ORIGINAL: RoswellGrey
That sounds like you have the MP3 player turned up too loud. Mine's the same way. I always have to turn down the radio controls when I use the MP3 player. I keep the player itself up loud because I have a little set of speakers for it to use in the house. For them, the player has to be set on LOUD.
That's my guess too - just a little odd that it never happens in any of my other cars, their volume overrides the MP3's volume, but for whatever reason they don't in this car. Oh well, if that's the biggest complaint I have...