Obtaining Free GarageBand Sounds
I like nearly all of Apple's $99 Jam Pack add-on sound libraries for GarageBand (Voices doesn't do a lot for me), but I also like free. And when I think free and GarageBand I think SoundFonts, E-mu Systems' sampled sound file format--a software instrument format supported by GarageBand.
Here's how to obtain and install SoundFonts:
1. Google "soundfont" to find one of the many sites that offer free SoundFonts--HammerSound is a good place to start.
2. Once you've downloaded a few SoundFonts, place them in this directory: /Library/Audio/Sounds/Banks.
3. In GarageBand create a new Software Instrument track, click the Details triangle, and from the Instrument Generator pop-up menu choose DLSMusicDevice and click the Edit button next to it.
4. In the window that appears, choose your SoundFont from the Sound Bank pop-up menu to play that sound.
Note that not all SoundFonts are compatible with the Mac OS. I've also found that GarageBand is happier if the SoundFont extension is spelled .sf2 rather than .SF2.