What do I need to look out for when selecting a relay for switching audio channels
The relay should actually be one of the most robust things in the circuit.

Make sure the coil is DC and not AC which could inject 60Hz noise.

The only real issue you might have to worry about would be one signal bleeding into the other--I'm pretty sure there is no spec on relays for this.  However, most relay contacts have pretty sizable air gaps (lots of isolation), and the bigger the relay, the bigger the gap.


Good luck.