It's REALLY time consuming to have to turn the volume down every time I want to scan source footage with audio. If you forget, it blares noise at you and you clients. Ever shoot in a factory or loud crowd or sporting event?
How the (@$#% hasn't this been added already?! It's such as SIMPLE request, and it's so easy to understand the benefits of this! While you can mute the Source Monitor using the Audio Clip Mixer, that window takes about 1000x more screen real estate than a tiny mute icon in the Source Monitor would. Furthermore, if the clip has several audio channels, it should be possible to mute individual tracks so that you can focus on the track that has the relevant audio.
I too, have wanted this feature since I started editing 5 years ago. "Its probably in the next update" - I'd tell myself. Now I find myself in the fetal position rocking back and forth in the shower until this gets added.