After considerable poking around, I still don't know why I'm hearing the weather station (or why it was the Spanish dub of Spidey 3 that one time), but I have identified a pattern and a workaround. It turns out that a random sampling of the stations I tune in (which form a proper subset of the stations available through my Comcast subscription -- my channel list excludes stations I never watch) put them into four categories:
- some have a single audio track;
- some have two audio tracks, with no sound on the second track;
- some have two audio tracks with the same contents on both tracks (at least same to my ears -- if one is higher quality than the other, I'm not detecting it); and
- ABC (only) has two audio tracks with that weather station on the second track (except, apparently, when it's something else).
The pop-up menu (triggered by the 'M' key on the keyboard or the 'Menu' button on the remote) has a submenu to control audio during playback, and for ABC I can use that to switch to the first track. It's a mild annoyance, but nothing serious. Searching online yielded other concerns about MythTV defaulting to the highest numbered track, but no solutions other than manually switching. So for now, at least, I will just have to live with switching audio tracks whenever I replay a recording of an ABC show.