I happened to have spilled coffee on my phone, and now the call sound isn't working right. Before, there was no sound (no speaker sound) and i waited until everything dried out and now there is speaker sound but the ear piece sound is very very low.. When ever i go into the settings -> sound and turn the sound on, the sound only stays turned up for like 10 seconds and it goes back to 1/4 of the bar.
from cydia, there are mods that you can download, which boost the overall sound volume of your iPhone, apart from any hardware modification, this may be the best way to fix your problem!