I have had similiar issues with my surround sound, part of the reason why i dont use it.
What i have expierenced was inconsistant levels of sound. Example: Im on the map boneyard and running around outside, the volume seems to be VERY low and i have to turn it up to hear. Then i run inside the boat and its just way too LOUD!!! I dont get why this is happening, but i had to play with the sound features in the online menu till for some magical reason, the volume levels out.
I would like to hear some solutions people have come up with to solve this problem.
As for what i am using for a stereo, its a technics reciever with an optical connection to my ps2. I will provide a picture as well below...