Audio dropouts without accompanying video freezes or pixellation are rarely "broadcast HDTV" issues or reception issues, and it would be rare indeed for a station or network to broadcast anything but the briefest of audio dropouts unless it was GIGO, and QC keeps that to a bare minimum. There are virtually no errors in encoding, which means virtually all errors are decoding (local STB) or reception errors. Reception issues typically manifest with audio and video issues that occur at the same time.
Generally audio-only dropouts are a local problem with reclocking of digital audio and are not related at all to reception. Often this can be fixed by upgrading the firmware for HDMI in the AVR. Optical seems to be more problematic than HDMI or other copper digital connections.
Stations broadcast either 2.0 DD or 5.1 DD as AC-3. That is also the only audio they send, meaning that PCM is reconstituted at the STB. That implies that if there is a drop out in DD that is not there in PCM, that the problem is most assuredly not a broadcast, transmission, or reception problem.
But, if the problem occurs only at the transition from network to local, that might indicate that the AC-3 bit rate is different from local to network. They should really match to keep the decoder from muting at the switch points. That particular issue will manifest on PCM as well, but has become somewhat rare now that most stations have either complied or use 5.1 generators so that they either send real or simulated 5.1 at all times.
If you switch to DD 2.0 in your AVR, you are really getting the same audio as 5.1, but remixing it to 2.0. Bass management is removed (bass below the LFE cutoff is mixed back in) and dialog Center is mixed evenly to L and R along with stereo music and FX, and the rear channels are phase-inverted and added back into L and R also. So it is really just a rematrixing of existing 5.1 channels (of course many AVRs will sense the change to 2.0 and then invoke Pro Logic or other methods to simulate and redistribute the audio to all channels so that your audio does not simply collapse into L and R). But that may be a clue; if you get dropouts in a 5.1 broadcast and change the downmix to 2.0 (in the same 5.1 broadcast) and don't get them, possibly that points to the firmware in the AVR. Hard to know for sure, but that would be the educated best guess.
Certain AVR models are notorious for this. Denon (a truly great brand) comes to mind. But owners sometimes interpret that FACT as a personal denigration of their purchasing intelligence when this gets reported, so don't be surprised if this brings the trolls out from under the bridge pretty quickly.
