There are now to distinct methods for resolving the Garageband 8bit error problem and other audio errors. Which one is write for you is dependent on version of Garageband you are running.
If you are running Garageband 10 on Mac osX 10.9 then you should go to the Garageband 10 fix page using the link below.
Click on here to link to the new Garageband 10 on Mac osX 10.7, 10.8 and 10.9 system.
There are a number of videos on this webpage you will find of use and a link to purchase a cheap 'starter' copy of the software.
There are also videos on using the Garageband 10 version on the following blog pages:
Garageband 10 Part 1
Garageband 10 Part 2