Petar,
The registry has the settings for all things for the phone. Incoming call volume, bluetooth, etc etc.
That is why you see all the hacks pertaining to modifying the registry. since I did have good volume on my phone BEFORE I upgraded to some ROM versions, I am pretty sure it has something to do with the registry. Now, even going back to the original ROM does not solve the volume problem. Although that does indicate that it couldnt be the registry, but I would like to knock on wood and see if I get lucky if some things inthe registry are missing for some reason.