speaker problems ??

my set up is a sony stereo receiver, bose speaker system, and fios cable and Panasonic plasma TV ......i've changed the cable and receiver and tv once each but my problem continues. While watch tv.. the sound will either cut off or lower...or the voice will cut off but the background music continues at the same time.... To fix i can raise volume to max or hit the receiver till the sound comes back on... Why this is happening i don't know. i've check the speakers and wires but don't see anything wrong . Any help would be appreciated. thank you.