NewSoftSerial rmp3_serial(6, 7);
NewSoftSerial rmp3_serial(3, 4);
mowcius - sounds like either you're listening to a VBR mp3 file (wide range one too! 32 kbps -> 320 kbps) and the music has really quiet and really loud spots, or your eyes are fooling you! The data goes directly from the card to the decoder. So, the only effect you'd see is if the MP3 frames had different bitrates.
Is there any easy way to get audio values sent to the arduino so a visualiser could be done from it?