Trying to get an old DVD/VCR combo player to work so our grandchildren can enjoy our library of Disney VHS tapes. The old RCA DVD/VCR combo is now connected to the Vizio TV using the composite cables and a coax cable since the back of the RCA unit says the only DVD/VCR output is thru the coax. The label above the component output states it is for DVD only. I used the Comcast Remote Control to switch the input source to "AV". I can now play a DVD using the RCA unit with no problem. However when I play a VHS tape I am only getting the picture, no sound. Any ideas?