Another option is to surface mount some speakers on the top gunnel of your boat and point them to the back. We have some Exile HLCD speakers that can be mounted that way. They are pricey as others have mentioned, but worth the money if you want crystal clear sound to the rider and beyond.
If you are serious about wakeboarding, you will want to consider a tower anyways. It is much more enjoyable boarding behind a tower, less fatigue and quicker ups and more tricks.
Come take a browse at your options when you get a moment on our website. Need some advice, give me a shout.