Radio problem

I have a 2003 E350 pasenger van with automatic transmition and when I put the van into gear the speakers shut-off. This doesn't happen every time, but is really annoying when it does, and it probably happens 40% of the time. When I put it in park or neutral the radio works fine. It appears the radio is still on, but no sound through the speakers. Any idea on what the problem might be and how to resolve it? My family is really getting tired of me singing to them on our 30 minute drive into town.

i have the same problem, anyone have any ideas?