A one click mute on event level would be nice, but in all audio situations I simply grab the top of the event (you'll see the cursor change) which activates the gain level, and bring it all the way to the bottom. Video events can be handled the same way (you get the opacity level instead), which can work as a mute, except in certain compositing modes.

And you also have the right-click option for muting events also. I believe multiple clips can be selected and muted in this way (but don't hold me to that until I check - memory isn't what it used to be).

Adding an envelope and then the nodes necessary to mute the audio or video object is probably more clunky than navigating two layers down to the mute button.
I think a mute button at the top level would speed up workflow. It exists in the audio software I use and its very handy.

You don't need to add an envelope - there's already a default control on each event, which as already described, you can just grab from the top and drag down to mute an item's audio.

yes but doing it that way doesn't retain the previous value of the object.

You can add a bind that calls right-click mute or even custom script with mute/mute unmuted/unmute of selected events