Dictation not working when trying to use mike in Studio Display -- works fine with M1 MacBook Air built-in mic

Dictation works fine on my M1 Macbook Air. But dictation does not work when I connect it to Studio Display and change to the Studio Display microphone in settings. I can see in sound settings that the the mic is active when i speak. Any suggestions for resolving this issue gratefully received.

Thanks

[Re-Titled by Moderator]