After many many tries I found out, what the issue is. I have tested different mp3-files on different tablets with different methods (USB, WiFi), sometimes the mp3 upload was successful, sometimes not. And I found the issue, why. Every time a mp3 file has no "interpret" set, but only "title" in the meta data, it was uploaded but not seen in MobileSheetsPro.
Either since companion v4 or MobileSheetsPro 3.9.1, it seems to be obligatory, that the mp3-file must have a title and interpret set in the mp3 meta data. All my missing mp3 could be now added without problems, since I wrote an "interpret" into the meta data.
When I import the mp3-file directly on Android from local storage, it's possible without the "interpret" meta data set. But with companion it's not.
Either since companion v4 or MobileSheetsPro 3.9.1, it seems to be obligatory, that the mp3-file must have a title and interpret set in the mp3 meta data. All my missing mp3 could be now added without problems, since I wrote an "interpret" into the meta data.
When I import the mp3-file directly on Android from local storage, it's possible without the "interpret" meta data set. But with companion it's not.