Could be the way your soundcard/speakers are set up. I was having a problem with a (midi) background sound drowning out other sound effects, until I realised Winamp had played around with my global sound levels. Have you checked the settings on Windows 'Volume Control'? (I'm assuming you're using Windows, but if not I imagine there'll be a similar thing in whatever you are using)
Otherwise - how big is the mp3 file? Could you upload it, and a sample room, so we can see if it happens to anyone else?
Otherwise - how big is the mp3 file? Could you upload it, and a sample room, so we can see if it happens to anyone else?