Do you notice a small or a big difference between the top and the bottom speaker?
I have the same problem, and it's very noticeable, even disturbing... The problem is you can set the balance more to the right or left in the setting but you'll have to constantly keep your phone in the same side, to avoid mixing right/left speaker.
Maybe the solution is gonna be a jailbreak and a tweak ^^
Anybody else has this unbalanced speaker problem. Is it general or a default impacting a few device?