The answer is already listed above here. For the DTA adapter - press the Lang (Language) button on the remote. Here's what happened - that same button got bumped and it switched the language to "Spanish." Some stations that don't broadcast in Spanish simulcast English of both audio/language streams --- and some do NOT. So-o-o you get audio on some stations and not on others.
Press the Lang button and switch it back to English and the sound comes back.
Related to this SAP issue is that potential that the audio receiver that routes sound through the optical cable from the DVR or DTA to the speakers could be piping different tracks. This was the case for me as I must have inadvertantly switched which audio track was playing. In my case, I switched to the Spanish track and could only hear sound on some channels with the Comcast box set on Spanish. I'm not exactly clear how this all works, but when I reset the receiver audio to its normal mode, all sounds worked fine. A generic approach here is to cycle through the receiver audio settings in case that is a source of the trouble. Good luck to all. This can be a real buggaboo.
Check and make sure SAP (Secondary Audio Program) isn't turned on on your tv. It could be a button on your tv remote but may have to be accessed through the menu. The button probably won't say SAP but might say MTS. If it's a Motorola cable box, go into the menu, settings, audio settings and make sure it's set to channel default and not spanish. If there isn't a second language on a certain channel or show, sometimes it will not have any audio on that channel or during that particular show. If that doesn't work call and schedule a service call