VoIP sound quality diminished during file downloads ?

If you overrun the available bandwidth then yes it'll potentially be altered or even interrupted. You can help mitigate this by using a VoIP gateway that has the first connection to the cable modem and uses QoS to make sure it always has enough.

Yeah, my wife was...well, not happy...when I would move Torrents around while she was on the phone. So I implemented traffic shaping. Life is much better now.

The Linksys WRT54G (with OpenWRT) works well for this if you just want something small and simple (and cheap).

--kyler