GarageBand 10 needs the loops and instruments from Garageband ´09 or ´11 installed to be able to play the older projects with all sounds. Otherwise instruments and loops may be missing. If you uninstalled GarageBand completely, reinstall iLife '11 to be able to use the older instruments and loops in GarageBand 10. before you install GarageBand 10.0.
GarageBand 10. comes only with a basic set of instruments and needs to be expanded by an in-app purchase. If you have GarageBAnd '11 installed, you should be able to get the in-app purchase for free. Expanding GarageBand for Mac (v10.0) content with an in-app purchase