Hello, JensBühl
Welcome to the Microsoft Community.
Thanks for visiting Microsoft Community. Hello, as far as I know, you should be an experienced and professional user.
I wish I could handle your problem, however, the issues on Mobile Speech Synthesis is out of reach of the response support community. It is more suitable for publishing on Microsoft Learn (English only), you can click on "Ask a question", there are experts who can provide more professional solutions in that place. Here is a link to the forum where you can raise specific scenarios and share your idea to help solve the problem.
Microsoft Edge - Microsoft Q&A Sincerely hope that your question will be dealt with appropriately after contact the correct department. Thank you for your understanding!
Additionally I've also taken this issue to feedback, if I have any new progress I'll keep you posted here, thank you for your understanding and cooperation!
Thank you for your time and effort in resolving this issue.
Best regards
Bobhe | Microsoft Community Support Specialist