Sound issue with z21

[Eric S+13]

I got a z21 Start for my Marklin HO 1928 Rheingold set. After several months of error free running the cars no longer make any sound when the coach lights are on. I don't know why turning the coach lights on and off interrupts the sound. It did not for the first few months so something has changed. I can actually start a conductor announcement and cycle the lights and the announcement will begin and end just muted for the second or so the lights are on. Any suggestions would be appreciated, to either fix the problem or determine if the problem lies within the train or the z21.