Audacity Forum

I think I see what he did. You may not like this arrangement as much as you did the old one. Leave Audacity off. Make sure everything is connected normally. FireWire, etc. but don't actually do anything to the Mac yet.

Mixer:
Turn all the FX controls down.
Turn the AUX RETURN down.
DO NOT select CD/TAPE TO MIX
DO NOT select CD/TAPE TO CTRL

Set up the rest of the mixer controls for normal speaking/singing level and you should hear yourself in your headphones.
Select CD/TAPE TO CTRL. Your voice in the headphones should stop dead. That's the button you press to hear the backing track when we get that far.

At the Mac:
Apple (upper left) > System Preferences > Hardware > Sound > Input > FireWire Device
Apple (upper left) > System Preferences > Hardware > Sound > Output > FireWire Device
Close the Panel.

Launch Audacity:
Audacity > Preferences > Devices > Recording > FireWire Device, Channels 2 (Stereo)
Audacity > Preferences > Devices > Playback > FireWire Device
Audacity > Preferences > Recording > [X] Overdub, [X] Software Playthrough

OK.

Restart Audacity.

Load and play your backing track and Press CD/TAPE TO CTRL on the mixer. You should hear the track in your headphones. Then press Stop.

Record in Audacity and you should record your new voice and hear both your voice and the backing track in your headphones. The new voice track should be pure with nothing else mixed in.

Koz