Vlad Fair point of course — one could argue there is a pretty strong convention for this sort of "power feature" on macOS without super overt signifiers, but agree it may not always to particularly discoverable.
In theory a combination control could work, i.e. SF Symbol "speaker.1-3" for volume level, which you can mute via direct click, then a "down chevron" indicating a click-to-open-menu interaction to instead modify volume. IMHO that could work well giving more control than other browsers.
…BUT over all, the simpler implementation is simply to allow muting only, and just let users modify their Mac's volume instead of the app — of course that's basically saying "no deal" to the original feature request. 🤷🏻♂️