i'm not sure if this is right - someone will need to confirm it but when you are in xolox and have songs set to download the program is constantly searching for those songs to download them - especially if it hasn't found anyone yet, i think it stops searching when there's a buffer of 20+. that means that xolox is sending out queries for the files and recieving back information on whether the files have been found.
if you dont have files set to download then the same senario is happening in reverse. other people are sending queries into your program for songs and your computer is sending back results.
i think that this is probably why you are seeing such activity through your modem