Widows-Insider Feedback (Microsoft):

“Windows has several audio volume APIs – IAudioEndpointVolume, IChannelAudioVolume, ISimpleAudioVolume, and IAudioEndpointVolume. These APIs can be used to change the volume and/or mute state of the stream, app, or audio device.

In build 17063 a change was made to these APIs to have them return S_FALSE (1) if the requested change was a no-op.

This broke apps (like Firefox) which request changes (that may be no-ops) and then explicitly check the return value against S_OK (0).”