Audition 5.5 - Can't hear any track while recording

I'm just trying to do some simple vocal recording over a music track in Audition 5.5, and as soon as I arm a record track, none of the other tracks playback (I can't hear their audio). As soon as I unarm the record, they will play again. I'm using a Zoom H6 as the input and output via ASIO, but I've tried another audio device and its doing the same thing. All the inputs and outputs on the tracks are set correctly. I know Audition can do this very basic function - what am I doing wrong?