I have a 56k dialup modem. Why can't I listen to 56bps streams?
Because 56k modems can't actually do 56k. They do about 45k (about 4500cps) on most phone lines, even the better ones. In-built modem compression makes up for this sometimes, but mp3 is already compressed so this has no effect here. You could try using an accelerated serial port (which will also take some load off the CPU), but be warned - the speed increase is not great at all!