no audio waveform display in 0.7.7 SVN

I just built & installed Kdenlive 0.7.7 SVN rev. 4309. Everything looks good, but no audio waveforms are displayed, whether as part of a video track or as a standalone audio track. Sound works okay, the Audio Thumbnail option is selected, I removed the old kdenliverc, still no joy. Is this a known problem ? Please advise.

[Edit:] I opened the Configure Kdenlive dialog. The Audio Thumbnail option was checked. I unchecked it, checked it again, and lo, the waveform displays are back. So what's up with that behavior ?