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.
Make sure your source (cable box?) is set to "stereo" and not multi-channel.
What do you mean by "hit the receiver"?
MY choices we mono, stereo, and surround... I had it on surround, per your response, i will switch to 'stereo'..... Hopefully this fixes the problem, i've been living with this way to long. Thank you in advance for your help.
Yes, let us know...

P.S. Make sure your interconnects aren't plugged into surround channels...