Hello, thank you for reaching out to Microsoft Q&A.
This problem appears if the system does not detect your default audio input device.
To make it detect it, you can try updating or reinstalling the audio drivers once and see if it works.
1
Open the Device Manager.
Expand the arrow next to Sound, Video, and Game Controllers.
Right-click your default audio driver, e.g., Realtek (R), select Update device, search automatically for available drivers, and click Update.
If Windows could not find any latest driver,
2
Right-click the default audio driver, e.g., Realtek (R), select Uninstall device > tick the 'Attempt to delete the driver software for this device' checkbox and then click Uninstall.
Once uninstalled, restart your computer.
See if your Microphone is being detected.
Let me know if you still need more help.