Audio quality changes on it's own

Hi Everyone, Been the owner of a Seal Design for a month now, absolutely love the car although I have had an annoying issue with the audio. For example, today I was driving to work and music was fine, had it set at adecent volume and I had a good bass (my music is very bass heavy) I stopped at a supermarket to buy my lunch then when I resumed my journey the music had gone very tinny, as if there was no bass whatsoever, no matter how much I turned the volume up I couldn't get it back to how it was before. It's not the first time I've experienced this and I just wondered if anyone else had experienced anything similar and if they had found a way to resolve it? Also, does anyone know where you can check how much data you have remaining on the car? I ran out last month Thanks in advance for any help Dave Autos & Vehicles