Seems that I solved my problem.
I had been experimenting with the Inguz DRC plug-in for the squeezebox, but had had some problems with it. I think I recorded my sweeps too softly, so the DRC process lowers the volume too much. That meant I had to turn up my pre-amp volume a lot, which ruined the signal-to-noise ratio. Suddenly, all sorts of wireless interference that had been inaudible previously became really, really annoying.
Now, with the DRC plug-in, I can choose from about four different room correction profiles (Strong, normal, soft and NoCorrection), or alternatively I can switch it off, all through the remote interface. I chose to switch it off.
Well, yesterday I hooked up some cat 5e cable between the SB and the music server, and switched into wired mode. I still had problems with some recordings. Then I turned the DRC back on. Hey presto, everything was better. No weird speed changes in the music: that seemed to be an artifact of telling slimserver that I would be doing DRC but then choosing not to do any. And no wireless interference, thanks to the wired connection.
Chad