A very recent tip from the german thread on sound problems:
It seems, that for at least two users the problem was solved by using another app:
“Simple Dialer”
This app is available from F-Droid, which is another plus. Just follow the link.
One user has reported, that he hasn’t got any troubles for more than six weeks now.
I hope this will help at least some of you too.
If that should indeed help, please report it to support as well, as this might help finding the root cause of the problem.