You can enable microphone and set audio redirection by customizing a host pool's Remote Desktop Protocol (RDP) properties. This lets you deliver an optimal experience for your users based on their needs. You can customize RDP properties in Windows Virtual Desktop using the -CustomRdpProperty parameter in the Set-RdsHostPool cmdlet. See supported RDP file settings for a full list of supported properties and their default values. audiocapturemode:i:value property RDP setting Indicates whether audio input/output redirection is enabled.
Source: Azure Documentation.