I called Surface Support and a guy named Mitchell talked me through the problem.
First do a refresh and that will take your Surface to the previous firmware version. Then do a windows update, and download all the new updates (it will take a while - mine had 22 updates). Make sure your Surface is plugged in and can access the internet the whole time.
Once the update is complete you will be able to edit the bass/treble settings via the sound control panel. Sound is much louder and better now - very happy :)
He said it was due to a an incomplete installation of the update...
I have done that at least 3 times. And no luck.