Garage band amps suddenly not working

Hi. I've been trying to solve this problem for two weeks, maybe someone can help. Just got off the phone with AppleCare, who were pretty useless---they want $49 to help me fix a piece of software that costs half that. Thanks, Apple.

I am running Garageband '11 through an M-Audio Fast track unit into an iMac OSX 10.6.8.

Two weeks ago, the amps in GB stopped working. All I hear is the sound of a clean electric guitar no matter what amp I choose.

The bizzarre thing is on every song I recorded before two weeks ago, if I go into the file, the amps all work.

But every time I create a new song now, the amps do not work, nor do I see a signal to tune my guitar through GB.

Anyone---anything - please - I have gigs and need to make demos for my musicians.