Hello,
For Microsoft Remote Desktop macOS, under "Device & Audio", ensure the checkbox next to "Microphone" is checked. This will allow your local microphone input to be redirected to the remote computer.
On the remote computer you're connecting to ensure that the system's sound settings are configured to receive and process audio input from remote sources:
If you do not see such an entry and you've already enabled microphone redirection in your remote desktop client, try disconnecting and reconnecting to the remote session. Sometimes, the device only appears when a remote session is active.
Also check on the remote host the group policy:
Computer Configuration > Administrative Templates > Windows Components > Remote Desktop Services > Remote Desktop Session Host > Device and Resource Redirection
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