Question [Q] Xperia Ion - Sudden Low Incall Volume

Hello to everyone.
After testing one custom ROM (Carbon) and while everything was fine, suddenly one afternoon the in-call volume lowered very much, so that I can't hear in a noisy environment. I don't know if the problem was caused by the ROM, I just tell the history. Then, I tried 2-3 other custom ROMs hoping that the problem will be solved, but nothing happened. Finally I got back to the stock ROM 4.1.2, (full wipe, data, cache etc. everything) having the same hope. But unfortunately, again nothing happened. I have also tried various volume control apps, but can't restore the max in-call sound.

In stock ROM, I checked the earphone in the service tests and the sound seems perfect, so i assume that it is not a hardware problem, but I cant be sure. Is there any possibility that this is a software problem? Does anybody have in mind any solution or idea to try? I checked 2 threads for a similar problem, but couldn't find any.