You could just CONTROL-click on that sound clip in Soundtrack. If the piece didn't originate in Soundtrack Pro, then first choose "Convert to Looping". Then CONTROL-click again and choose "Transpose". C is three half-steps from A, so choose "+3".
To change the key of the entire project, the drop-down menu should just be at the top of the timeline window. My version of Soundtrack Pro isn't the latest, so it's possible I'm missing something.
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