I do not see the waveform of the sound file

[support]

Waveforms are displayed only with Wave PCM file format.
You can for instance save your sound file with the free software "Audacity", with the default save file format (WAV).

Because the max authorised width for the picture of the wave (with zoom max) is 32000 pixels, the max duration of the audio file to see the wave is around 10mn.