In Multicam sequence only one audio track is audible?

Hi, I'm new to editing multicam sequences and am just working in the timeline for now.

I have synced four cameras and three further audio tracks, which has worked really well. The multicam sequence has muted all the camera audio, which is fine. However I want to be able to hear and edit all three audio tracks. These aren't muted, and yet I can only hear one of them - even if I solo the other two they are inaudible.

Would anyone have any idea what this is? I am sure it is probably a simple setting I'm unaware of but it has me baffled!

I'd be very grateful for any hints.

Many thanks

M.