where are you chartering out of? I've booked a trip out of St Martin in June on a Sunsail 39 and had the same question. I bought an inexpensive FM transmitter off amazon ($7) and it seems to work ok (not great but probably acceptable for me). I'll take a cord for an aux input and throw a couple of cd's in my bag just in case.
BVI... My Sirius xm radio had a built in xm transmitter, which is nice.
I found a nice griffin xm transmitter for the apple devices that has the transmitter in line with the 12 volt plug.