as Audioguru said it's probably student's design!
(if i could design too, it would be great! even at the same level!)
well....about the problem...i don't know what to say to you chemelec today i tryed external power supply unfortunately i don't have a regulated one... and the noise still exists! the board has a TDA2822m for the satellite speakers and the other one is also a TDA but i don't remember which one, it has heatsink and it is a bit diffucult to remove it. But i know that it is bridged so it can output about 7W RMS for the sub-woofer.
I repeat the problem is not so important, i just want to fix it just for education reasons...so i can learn repairing things.