[LAU] Distorted sound

I am running Qtractor and QJackCtl on a Raspberry Pi 3B+, with sound output through the headphone jack. When I play a wav file using 'aplay' it sounds fine, but if I load it into a track in Qtractor, it is very distorted. A female voice sounds like basso profundo (although the speed is correct) and is very distorted. It looks to me as if this is probably because the output of Qtractor is not being directed through jackd. QJackCtl seems to show that jack is running OK and speaking nicely to Alsa, but there is one oddity. The Qtractor Connections window shows, in the Audio tab, the expected Qtractor output connected to System. Normally I would expect the QJackCtl Connect window to show the same in its Audio tab, but in fact it shows nothing at all - neither Qtractor nor System. (Yet it is showing the expected Midi Through and Qtractor on both sides of the Midi tab, cross connected as expected.) The Messages tab of QJackCtl shows that jackd is running, but it doesn't report anything extra when Qtractor is started. Yet on my Ubuntu machines, QJackCtl knows automatically when I start Qtractor. Any ideas why QJackCtl doesn't seem to know what's trying to connect through it? David