Very common problem

This has become a nightmare for those with pre-eisting home theater systems that are analog and have delayed getting a new A/V unit to come up to the newer HDMI standard. Mixing analog/composite signals from the cable box (does it have an HDMI jack?) and inputting into both an analog amplifier and the TV too. Conversion can be expensive and often doesn't pass along all the decode info for the different formats. Pay close attention to any optical connections as they offer all the decoding and are more full spectrum and may be pressent on some older A/V units. This would provide the audio without a sync problem with converters.