Get an old couch hull. Modify it into a shuttlecraft for your stand up. Something like a Yamaha GP hull. Wire it for sound and run it off a boat battery. Charge the battery back up at the end of every day. Set your couch-craft up so it carries a cooler full of cokes, an anchor and some bouys in addition to your sound system. You could even craft the nose or sides of the ski into seats. Drop anchor, detach, ride your stand up as a stand up, reattach and cruise back home.
Putting speakers into a race ski would be bad enough. Adding all that weight to a freestyle ski would completely defeat the point. If you absolutely insist on doing this to a ski, use a WaveBlaster 1 or a Seadoo HX. They'd be a better platform for a sporty jam.
Not trying to be a dick, but if you're riding a stand up and afraid to get your ears wet, you might be riding the wrong boat. The Wave Blaster 1 and Seadoo HX are really sporty fun to ride boats that will keep your head out of the water and give you the crusing conditions you are seeking. Can even ride 2 up.
You can do anything you like. You can put speakers into the stand up. It just defeats the point of owning the stand up. If I were going to do it, I'd either put them in the hood facing towards me using bullet connectors, or in the sides of the tray facing my feet or slightly up, if I wanted to do more than just cut a round hole. In the front of the tray facing backwards will not get you much sound.