That depends - do you want to record STREAMING AUDIO (from the internet), or just audio from a CD you're playing for example?
If you want to record STREAMING audio, you can't go wrong with
WM Recorder - it's able to record Windows Media Player files, Real Player files, even a bunch of flash (rtmp, flv) type files as well.
If you want to record the audio from a CD on your PC - you should check out
Replay Music which was made just for this sort of thing.
Try em both (free demo) and see which one bets suits your needs.
Let me know if you have any questions, I use them both and love them both.
MG