Audio element: need ducking when Hubs video is playing

[j-conrad]

Suggestion for the Audio element: A checkbox (perhaps with a slider for amount) that would make that audio source automatically duck when a video is playing within Hubs.

Currently, it's nice to be able to add background music or ambient sound. However, it can interfere with someone pasting a video in Hubs (or playing a pre-placed Spoke video), especially if the controls are disabled for the audio source.

┆Issue is synchronized with this Jira Task

[robertlong]

I think this should probably be discussed in Hubs as all media will probably want to have this feature if we decided on implementing it. I'm going to move it over there.