depends how much you want out of the sound unit really, I have a technobots mp3 sound card, as developed by Alan Bond of the solent club, and as featured on the technobots site.
I use a mini sd sound card with a variety of downloaded sounds, engine, and dialogue, music, etc from the jaws movie, as its in the Orca Jaws boat. Alan is a really clever guy, he made me a commission build way of selecting tracks from my transmitter, using a spare channel via one of the sticks, so many prods, selects a certain track. When I finish my PCF vietnam boat, I will just lift the sound unit out, swop the sd card, and then will have sounds, music etc relevent to that boat.
We did this before that sound unit was available. Its over the top, but something I really wanted, makes sailing the boat good fun, there is always someone who starts singing the jaws theme, and its a nice touch when I can play things from the film at my command, not just on a loop etc. THe unit is really very versatile, as I can put anything on the sd card, and play it at my request, I have stereo mini speakers, and a small amp from a set of pc speakers, hope that helps