I have one track in the mix already (my original singing), and I'm trying to record the second track in the mix. However, I get a shocking level of hiss and the audio is muffled and distorted, as if it's picking up interference from somewhere.

I've muted Track 1 while I'm recording Track 2 - I don't need to hear the original audio, I just need to see the timecode in order to match the timing of the original track - so that shouldn't cause any inference. Any ideas?

Ah, solved my own problem. The program was picking up monitor input, despite this option being definitely unselected. Closed and reopened, sorted the problem. Yet another weird AudioDirector glitch...