[wdfifteen]

When I got home I looked up the make and model and started doing some diagnostics. The problem is Time Warner - their cable or their cable box. I thought the scratchy, burbling sound was present with all inputs, but the other observers here say it is only there with input from the Time Warner box, not the DVD player or Apple TV. It is DEFINITELY worse with the Time Warner input. I switched HDMI cables between the Apple TV and the Time Warner box to eliminate the cable as a problem - no help.

As for the reason for the optical cable between the TV and the Onkyo HT-S5100 AV receiver, from Onkyo's website:

" ..as in the manual for the HT-S5100 on page 36 under the AUDIO SIGNALS:

• If you want to listen through the speakers connected to the AV receiver, in addition to an HDMI connection, you’ll also need to make a separate analog or digital audio connection.
This receiver utilized an early version of HDMI which only support HDMI video and not audio- so you will need to also provide a separate audio stream; unlike our later models that support HDMI audio and video."

Thank you to everyone who rang in with suggestions for fixing this.