Are you using the spray in wet cellulose or the dry stuff? I've heard th dry stuff isn't worth anything for sound proofing. Sure it will fill in the void and have some effect, but you might not notice much of difference. Once you blow that stuff in you're not going to get it out without putting some big holes in the walls.
Blown in fiberglass would be much more effective, both from a heating & cooling and sound proofing nature.
Take a look at >>
this stuff <<. It might be worth investigating if you're trying to squeeze every single last STC point out of your wall. Check out the rest of the site too. There's a lot of info about sound proofing there.