it happened to me also, as radio mundu uses phones main memory for buffering and its not cleared while changing the stations it happens, for me it solved by formating the phone and reinstalling the radio mundu software, but this is not the correct way,I think phone geeks here will give you better solution
I think formatting won't solve da problem. My frnd face da same problem in Nokia N73. He formatted phone & reinstalled the software but problem persists.
Anyway I don't wanna format my E51. Does anybody has same problem & and idea to solve?????????