We had the jazz trio at the LS and planned in advance for the 20 minute breaks. We burned CDs and just gave them to our coordinator before the wedding. We did not need to bring our iPod or speakers. I would recommend going the CD route versus trying to find speakers.
However, I would also note that the 40 minutes on, 20 minutes off wasn't really accurate. They seemed to play for longer than 40 minutes, and then take the 20 minutes. It didn't seem scientific, anyway. Just be flexible - they will play their piped in music during the breaks if you don't give them anything, and we had to remind them to actually start our CD playing during the break.