Check C131 which should have 5 volts across it. This powers the analog switch chip that has to select the audio channel and routing. If 5 volts isn't there, check cap for a short. There are too many other possibilities to enumerate here. You need electronic test equipment to troubleshoot this problem and some skill at troubleshooting.

Here is your schematic:

http://elektrotanya.com/marshall_mg50dfx_sch.pdf/download.html