Kdenlive/Manual/Effects/Misc/Audio Wave
Audio Wave
Display the audio waveform instead of the video. Author Dan Dennedy
Overlaying the Wave
This effect replaces the video. If you want the effect overlaying the video you can do something like shown below.
Duplicate the video track on a track below the one with the Audio wave on it.
Add a composite transition.
On the top video track (the one with the audio wave effect) add a color selection effect.
Make the color you are selecting black and check the invert selection.