Finally did it! My mistake and misunderstandig was, to implement the camera back to homekit via the Hass source like:
streams:
generic_camera: hass:Camera1 # Settings > Integrations > Integration Name
aqara_g3: hass:Camera-Hub-G3-AB12
But the solution for me was now:
streams:
klingel:
- homekit://...... (Homekit link taken from the "Add"-Tab under Home-Assistant)
- ffmpeg:klingel#audio=opus
homekit:
klingel:
But… after the pairing it worked fine, 2-way-audio also worked…but just once. after I closed the feed and tried to reopen it, it says “ne response” 
ahhhh!
Here is my log:
14:27:33.088 TRC [homekit] 192.168.1.110:51466: get pair id=371A5075-658B-42CD-8F0A-81A397ACF7AB
**14:27:33.088 ERR github.com/AlexxIT/go2rtc/internal/homekit/homekit.go:174 > error="hap: PairVerify from: 192.168.1.110:51466, with unknown client_id: 371A5075-658B-42CD-8F0A-81A397ACF7AB"**
14:27:33.096 TRC [api] POST /pair-verify 192.168.1.110:51467
14:27:33.101 TRC [homekit] 192.168.1.110:51467: get pair id=34496E25-61E1-4047-AA13-B7B542E74B54
14:27:33.817 TRC [homekit] 192.168.1.8:49482: add pair id=E7E25DC4-4190-4B1B-A8EB-BDD3B079B581 public=26d44f9cf3e378793558971cf33f3a8a151ee54e21e03d7f915c01446b94f4c2 perm=0
14:27:33.817 TRC [homekit] 192.168.1.8:49482: get pair id=E7E25DC4-4190-4B1B-A8EB-BDD3B079B581
14:27:40.931 TRC [api] POST /pair-verify 192.168.1.122:49267
14:27:40.937 TRC [homekit] 192.168.1.122:49267: get pair id=34496E25-61E1-4047-AA13-B7B542E74B54
The IP in the error line is from my AppeTV.