Hi,
In order for microphone redirection to work, you need to have Remote Desktop Session Host (RDSH) Role Service installed, "Record from this computer" selected in mstsc on client PC, and the RDP session needs to be a regular session (not admin mode session). Once RDSH is installed on the server and you restart, the default will be to connect to regular session if you open up mstsc.exe (with no command-line options) and connect.
You will need RDS CAL(s), install/activate/install licenses on RD Licensing server, set licensing mode and licensing server name in gpedit.msc. If the server isn't domain-joined then only RDS Device CALs are supported.
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Thanks,
-TP