You may need a louder stereo, more speakers, or it may just be speaker
placement. (Not that the bus ain't noisy!) I had enclosed (box) speakers
mounted back by the rear seats on my '70 camper wired to a 40 watt Stereo and
it seemed loud enough! Later on when I got more cash I hooked up a pair of
6x9 speakers in the doors near the bottom and man was THAT kickin'!...Until
the stuff got stolen, of course. Not the one's in the doors though, I guess
they didn't see them. (Another reason to put them in the doors.) You could
also try the storage box underneath the  rear seat/bed which is what Hot VW's
did and they seem happy with it. I don't do it because I'm not into cutting
up my plywood. 

A final note though: I agree with and appreciate all the comments about
hearing protection. I'm also a motorcyclist and hearing loss is a big concern
after riding all those hours with wind buzzing around inside your helmet.
Now, if they could invent something better than foam plugs...

Mike
Mein Kamper:1970