So, I was tracking this issue a little bit more, and I noticed that my original assumption may not have been completely accurate. It doesn't seem to be tied to starting the car up, but instead to one specific station that I listen to regularly. It's a not-so-local station (and thus has a slightly weaker signal, but still strong enough for stereo signal, and no noticeable drop in quality 95% of the time). I got this idea from a little stretch recently where I would tune to this station (by preset), and the stereo would automagically tune itself to 103.1 FM within 15 seconds. It was a constant battle with the stereo, where I would tune it back to the non-local station, and then it would tune it back to 103.1 FM. Here's my observations so far:

- I've verified that TP is off for my stereo, so I don't think it's related (for now).
- However, I thought I remembered reading something in the manual about "diversity tuning" or "follow-me tuning" or something similar.
- This involves stations that are "tied" together in broadcast (this is used in remote areas so that, as you drive away from one station, you just tune to the other station which has the same exact broadcast). The stereo will figure out which stations are "tied" together (from a code in the broadcast) and automatically tune to the station with the stronger signal at any given moment.
- In this case, the broadcasts do not appear to be the same, but perhaps the code sent in the broadcast may be incorrect (or not valid for my region).

I haven't had time to dig through the manual again, but I think I read this diversity tuning stuff in there. I think there should be a way to turn it off, and hopefully that will make the voices stop.