I don't know why this is happening, but Vegas is importing some mp4 files, generating audio errors. This is manifested as cracking or short loud whistles. I tried to turn off So4 . This helped with one video, but with the other video from the second phone, it did not import the audio at all.
I had to open the mp4 file in the Reaper (DAW audio software) and render the audio separately.
I met this problem for the first time because these videos were not filmed from my devices.
The same problem is with VP15 on my laptop like VP16 on my workstation.
How is it caused?