Lee - The rMP3 only uses 2 serial pins to communicate. The SPI port is there as an optional way to control the rMP3, but the current version of firmware (100.01) hasn't got that implemented (expect it in the next version).
You figured out how to do the serial change though. Just cut the jumper at pins 6 (rMP3 TX) and 7 (rMP3 RX), and reroute the pads to the new pins you want to use. (make sure you connect wires to the pads FURTHEST of the two from the Arduino header).
The change in the code should be really simple. Wherever you see:
NewSoftSerial rmp3_serial(6, 7);
change it to (in your case):
NewSoftSerial rmp3_serial(3, 4);
(I'm assuming that you changed pin 6 on the rMP3 to pin 3, and pin 7 to pin 4).
If that doesn't work, post your code and we can see what's going on.
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.
RE: documentation, our wikidocs have always been public, unfortunately you had to register. I've changed it now so that you don't have to register. Just add any comments/questions you have on any page that has a discussion.
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