Problems with the sound

Hi there,
I've got some problem with the sound of my Xiaomi mi A3.
Since some month every time I start any audio, like music or video, it begins normal but after some seconds it disappears. Usually it took some time (about one minute) to recover to normal.
Since the latest Android update (Feb. 5th 2020) most of the time it doesn't come back at all.

Sidenote: With Bluetooth or headfones the sound is staying at the same volume.

As it became worse after the latest update and it is not with just one app, I habe the impression that the problem is at the operating system, not the apps or the phone itself.

Any one there who could help?