I've been re-watching the DVDs of Simon Pegg/Edgar Wright sitcom Spaced, and noticing that the volume levels go up and down all the time. I didn't remember that being the case from the first time I watched them… which was on a previous DVD player (a Pioneer).
Turns out I'm not the only one to have this problem — those reporting there noted it was an issue on some Toshiba players, but I'm currently using a Sony.
I also have a Panasonic Blu-Ray player I could use, but it's not multi-region, and these discs are Region 2. Damn. And I can't see any easy way of hacking the player to make it multi-region, alas.
That's okay though, because taking a cue from the forum led me to this post about DVD sound problems, and also to a Wikipedia description of Dynamic Range Compression — leading me to think this was causing the issue.
I found this in the DVD player's setup menu. Once I'd switched it from “TV mode” to “standard”, all seemed to be okay again, though in contrast, a newer DVD of Parks & Recreation now seems to have its volume fluctuate, so perhaps I need to switch it back for everything other than Spaced. Odd.