We are aware of an issue with Max5 and some sound cards on Macintosh under OS X 10.7, including built in audio. The sound from inputs appears bizarrely distorted and will render the audio in your patch unusable.
We are currently working on the issue in engineering. Expect the fix for the Max 5.1.9 update, or possibly an incremental download before 5.1.9 ships.
If you rely on your Mac's built in audio for sound we recommend you do NOT upgrade to Lion at this point.