It depends on the event you select first. If it is unmuted and then you add to the selection one or several muted events, you will be able to change the volume of them all. But if you select a muted event first and then add the unmuted events, volume will be unavailable. Definitely, a bug.
100%
Just checked, this behaviour started with 7.5; before that you could adjust volume on muted events. I can’t imagine it would have been deliberately introduced as an FR so I can only imagine that it’s carelessness. Recently, clicking on a event name suddenly stopped selecting the whole name. It was fixed. I think this is in the same category.
What I’m seeing is that it depends which selected event comes first in the time line. If the first event is muted then you cannot adjust volume in the info line. It does not matter what order you select the events in, or if you add further events to your selection. It only matters whether the earliest selected event on the timeline is muted.