Thanks Jason - no worries, subsequent mucking around has shed a bit more light on this issue. The EmuMovies database for music is far from comprehensive, although better for some platforms than others. In this case the platform I was testing with was Amiga 500, which has very little music on their servers.
However, turns out that when you're scraping an individual game, the scraper will match and download music that's from a different platform from the ROM itself. An example is The Adventures of Robin Hood, which released on both Atari ST and Amiga (among other platforms). I've got the Amiga rom, which has no matching music, but EmuMovies does have music for the Atari version. So that's what Launchbox was downloading. However, when doing media downloads for multiple games, the scraper is strict about platform.
Unsure why this is the case, but It's probably not intended behaviour. It would be nice to have the option (a tickbox maybe?) to do platform agnostic media scraping en masse.