I had really poor sensitivity on the radio. Pulling it to check -- and to install the Audio Connectivity Module I took out of another X-Type -- I found that there was an XM radio adapter that feeds to the antenna as an FM signal, inline with the original antenna. Removing that, installing the wiring and optical pigtails for the ACM and plugging the original antenna lead, I switched on the radio to test it and . . .NOTHING. No sound. Hitting the phone/mute button on the steering wheel, I got a MUTE ON notice, but it wouldn't UNmute.

I tried hooking up the power and optical lines to the ACM, and -- ta daa! Got my audio back.

The moral of the story is that if the optical cable isn't plugged in, the radio mutes, so that would be the first thing to check if the radio is silent.