for what it's worth, I doubt whether the Mac being on a desk or not would effect the sound of your recording. Titchy built-in mic housed in a plastic or aluminium container isn't liable to be picking up vibrations from much. If it sounds like you're singing into a bucket, it's more likely to be the room ambiance or reflections from a wall. (Or, of course, the fact that it's just the crap built-in mic!) Try moving the Mac to different places around the room (or try a different room)