There are myriad audio recording apps for the Mac now. Some are (kinda) built in like Quicktime Pro's "New Audio Recording" and some are expensive like Final Cut Studio's ability to record directly into Soundtrack Pro and Final Cut Pro.
A new FREE audio recorder is AudioCapture [app store link] It records to these formats:
But it does so as a .mov which while not a deal breaker is a bit of a hassle. Still, it's free.