Mary makes fun, forgive her. Some of this was discussed a year ago HERE.

My thoughts are that a small flash drive MP3 player would be better thinking. Your car decks and speakers would have to be waterproofed. You will need a 12V source (Where are you going to put and secure that battery? And it would have to be a fairly new battery to supply you with the juice for a couple hours sailing. Depending on your supplies, I don't think that you can provide, install and waterproof everything needed to do this on the cheap.
Do you really want a lead acid battery weighing 40 pounds on something that capsizes/pitchpoles?