There are lots of good, free virtual instruments out there. Finding exactly what you want is a lot more difficult. These are collectively called, "VSTis." That's Virtual Studio Technology Instruments." There won't be a test, but if you're looking for terms to Google, there ya go. I'm also not going to talk about VST effects, right now. There are also a bazillion of these.
I first got Kontakt Player. It looks like it's one of the more "standard" VSTs out there and there are tons of instruments built specifically for it. It even comes with a smattering of instruments:
Classic Bass
Clavinet
E-Piano
Funk Bass
Funk Kit
Jazz Guitar
Jazz Organ
Muted Trumpet
Pop Kit
Ragtime Piano
Rock Guitar
Street Knowledge Kit
Upright bass
Cross-platform, too.
I had two problems installing it. First, I had no audio. That was because I have multiple audio devices -- my internal sound card and an HDMI connection to my TV -- and the sound card wasn't set as the default. That's a four-click solution in Windows 8.1: right-click the speaker icon in the system tray (that's where the clock is), click Playback Devices, click on the device you want as default, and click "Set Default." Viola. Sound!
The second problem I had was that I kept on hearing a piano sound mixed in when I was playing with Kontakt's internal virtual keyboard. I eventually figured it out, which is good because I Googled and found some hits for, "Kontakt player keeps playing piano," but the solutions were for people that had keyboards connected. Mine wasn't. Solution: click the Options button in Kontakt, choose MIDI, choose Outputs. Set the Microsoft GS Wavetable Synth to OFF.
So, let's hunt for a clarinet and violin that sound good. It was surprisingly difficult to find a solo violin. Everybody seems to think that you need a string section. I ended up with this one. Clarinet was easier. I got this one. The violin set also has built-in pizzicato, which is used a few times in the Milhaud piece.
I haven't had a chance to do more than try out the sounds: finding good ones was no small feat and I'm working on a project for Helen, right as I type!
I give you linkies for free stuff:
http://www.native-instruments.com/en/community/user-libraries. Native Instruments' user-created sound libraries. They're the folks that make Kontakt.
https://www.applied-acoustics.com/swatches/overview. Mostly synth.
http://bigcatinstruments.blogspot.com. A little bit of everything.
http://www.buzzmachines.com. More synths and sound effects.
http://sso.mattiaswestlund.net. Orchestral samples for SFZ Player.
http://www.soundsonline.com. Commercial, but have free VSTs, occasionally. I think I got a free piano ....
http://www.vstplanet.com. A lot of everything. Generally has some stripped-down commercial products available for free.
http://www.dskmusic.com/category/vsti-all. Speaking of stripped-down commercial products, this is DSK Music's free section.
https://www.dubstepforum.com/forum. The Dubstep Forum. There are a bunch of folks that build and post VSTs for you to play with.
http://www.creative.com/emu/proteusvx. E-Mu Proteus VX Sampler VST. Also has some very nice sounds.
https://steamcommunity.com/app/222710/discussions/1/846965611318081705. A Steam forum that branched into VSTs and VST players. Quite a lot of stuff.
https://web.archive.org/web/20140208055725/http://freemusicsoftware.org. The Archive.Org page for FreeMusicSoftware.org. It is what it says on the tin. Closed in 2014.
http://bedroomproducersblog.com. Reviews and lots of freebies.
http://www.ivyaudio.com. A very nice piano and an interesting vocal sample.
http://www.kvraudio.com/plugins/newest. A bazillion VSTs and other audio plugins.
http://www.anthonydeaton.com/philharmonic.html. Small selection.
https://splice.com/plugins/free_plugins. Mostly effects, but some VSTis.
http://vis.versilstudios.net/products.html. Small selection of VSTs.
http://earmonk.com/free-sample-libraries. A link-list of sample libraries. Note that some of these are just samples, so you'd have to record them with a sampler to get them to work as a VST.
http://www.vi-control.net/forum/index.php?h=0&pf=0&c=15. Virtual Instruments Composers Forum. This is the direct link to the Virtual Instruments & Sample Libraries section of their forum.
Commercial Orchestral VSTi:
http://vsl.co.at/en/Products. Vienna Symphonic Library. Extremely high-quality orchestral instrument VSTs.
http://www.soundsonline.com/Classical-and-Orchestral. EastWest virtual instruments. Also very high quality.
http://www.spitfireaudio.com/albion. Spitfire Albion. Decent strings; I feel their trumpets are a tad synthetic-sounding.
http://www.cinematicstrings.com. CinematicStrings. Find out what you're good at and keep doing it. They're really good at strings.
http://audiobro.com. Audiobro LASS. Also a very competent string VST.
https://cinesamples.com. Lots of orchestral instruments. They also collaborate with Abbey Road Studios.
https://www.projectsam.com. Symphobia has some really nice orchestral groups. They also have a lot more demo content than some of the others on this list.
http://soundiron.com. They have vocal samples, too.
I first got Kontakt Player. It looks like it's one of the more "standard" VSTs out there and there are tons of instruments built specifically for it. It even comes with a smattering of instruments:
Classic Bass
Clavinet
E-Piano
Funk Bass
Funk Kit
Jazz Guitar
Jazz Organ
Muted Trumpet
Pop Kit
Ragtime Piano
Rock Guitar
Street Knowledge Kit
Upright bass
Cross-platform, too.
I had two problems installing it. First, I had no audio. That was because I have multiple audio devices -- my internal sound card and an HDMI connection to my TV -- and the sound card wasn't set as the default. That's a four-click solution in Windows 8.1: right-click the speaker icon in the system tray (that's where the clock is), click Playback Devices, click on the device you want as default, and click "Set Default." Viola. Sound!
The second problem I had was that I kept on hearing a piano sound mixed in when I was playing with Kontakt's internal virtual keyboard. I eventually figured it out, which is good because I Googled and found some hits for, "Kontakt player keeps playing piano," but the solutions were for people that had keyboards connected. Mine wasn't. Solution: click the Options button in Kontakt, choose MIDI, choose Outputs. Set the Microsoft GS Wavetable Synth to OFF.
So, let's hunt for a clarinet and violin that sound good. It was surprisingly difficult to find a solo violin. Everybody seems to think that you need a string section. I ended up with this one. Clarinet was easier. I got this one. The violin set also has built-in pizzicato, which is used a few times in the Milhaud piece.
I haven't had a chance to do more than try out the sounds: finding good ones was no small feat and I'm working on a project for Helen, right as I type!
I give you linkies for free stuff:
http://www.native-instruments.com/en/community/user-libraries. Native Instruments' user-created sound libraries. They're the folks that make Kontakt.
https://www.applied-acoustics.com/swatches/overview. Mostly synth.
http://bigcatinstruments.blogspot.com. A little bit of everything.
http://www.buzzmachines.com. More synths and sound effects.
http://sso.mattiaswestlund.net. Orchestral samples for SFZ Player.
http://www.soundsonline.com. Commercial, but have free VSTs, occasionally. I think I got a free piano ....
http://www.vstplanet.com. A lot of everything. Generally has some stripped-down commercial products available for free.
http://www.dskmusic.com/category/vsti-all. Speaking of stripped-down commercial products, this is DSK Music's free section.
https://www.dubstepforum.com/forum. The Dubstep Forum. There are a bunch of folks that build and post VSTs for you to play with.
http://www.creative.com/emu/proteusvx. E-Mu Proteus VX Sampler VST. Also has some very nice sounds.
https://steamcommunity.com/app/222710/discussions/1/846965611318081705. A Steam forum that branched into VSTs and VST players. Quite a lot of stuff.
https://web.archive.org/web/20140208055725/http://freemusicsoftware.org. The Archive.Org page for FreeMusicSoftware.org. It is what it says on the tin. Closed in 2014.
http://bedroomproducersblog.com. Reviews and lots of freebies.
http://www.ivyaudio.com. A very nice piano and an interesting vocal sample.
http://www.kvraudio.com/plugins/newest. A bazillion VSTs and other audio plugins.
http://www.anthonydeaton.com/philharmonic.html. Small selection.
https://splice.com/plugins/free_plugins. Mostly effects, but some VSTis.
http://vis.versilstudios.net/products.html. Small selection of VSTs.
http://earmonk.com/free-sample-libraries. A link-list of sample libraries. Note that some of these are just samples, so you'd have to record them with a sampler to get them to work as a VST.
http://www.vi-control.net/forum/index.php?h=0&pf=0&c=15. Virtual Instruments Composers Forum. This is the direct link to the Virtual Instruments & Sample Libraries section of their forum.
Commercial Orchestral VSTi:
http://vsl.co.at/en/Products. Vienna Symphonic Library. Extremely high-quality orchestral instrument VSTs.
http://www.soundsonline.com/Classical-and-Orchestral. EastWest virtual instruments. Also very high quality.
http://www.spitfireaudio.com/albion. Spitfire Albion. Decent strings; I feel their trumpets are a tad synthetic-sounding.
http://www.cinematicstrings.com. CinematicStrings. Find out what you're good at and keep doing it. They're really good at strings.
http://audiobro.com. Audiobro LASS. Also a very competent string VST.
https://cinesamples.com. Lots of orchestral instruments. They also collaborate with Abbey Road Studios.
https://www.projectsam.com. Symphobia has some really nice orchestral groups. They also have a lot more demo content than some of the others on this list.
http://soundiron.com. They have vocal samples, too.