I have problem with Samsung Galaxy Ace Duos during calls. I hear very clear but callers complains that I'm being heard very low. Sometimes impossible to hear me. Could it be regarding the fact that at the same time there are 2 SIM cards and they "fighting" somehow?
It has nothing to do with the SIM cards, I practically guarantee that
It could be however, your microphone is damaged or plugged in some way. I used to work in a factory building trailers, and at the end of the day my pockets would be filled with sawdust. This meant the headphone jack, speaker, microphone, etc on my phone were all full of sawdust too.
A quick blast of compressed air should do the trick.
If it doesn't help, the phone might be damaged internally. The wires that run from the microphone to the phones mainboard could be frayed or coming loose, and would weaken the connection and therefor the volume.
I'd take it in to get checked out and possibly repaired if hitting it with some compressed air doesn't do anything.