JJ market will be open, but there will be many, many shops which are closed.

I would strongly recommend staying inside during the day if you are in a popular tourist area and don't want to get wet. Every year I have tried a new tack for the festival (which usually runs for 4/5 days on KSR). I have tried the water fights, I have tried locking myself inside and all other combinations in between. I think as a tourist just head for a tourist area, put your phone in a condom, wrap your cash in plastic and away you go. Just enjoy. At night all the restaurants and shops are open. There is a good atmosphere, just stay away from the 'youths' on KSR who all get loaded with Lao Khao and like to fight.

As for MP3, do not go to Pantip or MBK. They are dens of thieves. Go to Fortune Town instead (Rama 9). It is much better and you will have less chance of being ripped off. I can't recommend an actual shop, but they are better than the places in MBK and Pantip.