Uli,
You mention, "It working after taking the box off the mount points to the connectors." and that you have carrier without any audio. I would suspect either the microphone, the mic wiring (in/out of the speaker box), or the mic connectors (male and female) at the Dittel box. Especially the connectors.
If this is actually a Becker box then you can look at the Becker 4201 manual on page 2-8. See
https://www.cumulus-soaring.com/beck...201-Manual.pdf
Finally, I created a troubleshooting guide that MIGHT (maybe kinda) help you. I just added a section on the Dittel/Becker boxes. See
http://aviation.derosaweb.net/presen...leshooting.pdf
Best of Luck, John