The latest Steam beta client (dated July 26 or newer) adds streaming microphone support on Windows 8.1 and Windows 10.
Once you install the beta update you may need to reboot your computer to activate driver installation.
Then, when you start streaming for the first time you'll be prompted to install streaming audio drivers on the remote computer.
When streaming starts, Steam will switch the default audio devices over to new virtual audio devices that are used for streaming audio, and restore them once streaming is complete.
If you want to avoid this completely, just start Steam on the remote computer with the -skipstreamingdrivers command line option.