SB3 won't play 48 kHz MP3s anymore

As the title says, they appear to start but no sound - then once it gets to six seconds or so it just restarts - so it just sits there playing nothing.

I know they used to work a long time ago and I know they don't work now (on some 7.9 versions including the latest nightly: 7.9.0 - 1465358341) but I don't really know when they stopped working because I don't listen to them very often.

I blame a server upgrade, but it's not impossible that I've messed up some config somewhere.

I don't see anything in the server log, but of course that's with default logging.

Anyone any ideas?

Ben