Mute option.
This will sound weird. But I like practicing with a piece while it's playing so I can read/play at the same time. But I don't want to be distracted by the music so I can hear what I'm doing wrong.
Turning off my speakers mutes my guitar, which I don't prefer. And muting the overall volume does the same since I'm plugged in to my computer. So the only true workaround is to mute just the player after I hit play so it shows up under Volume Mixer -> Applications. So it would be more convenient and a better reminder to have a mute option along with the playback toolbars.
It's probably not a common request/desire, so I think my one shot at this is that it would probably be easy to do. :)
When I understand correctly, you want to mute all instruments in the player. For me it makes more sence when I can mute every single instrument apart, so I can play along minus one (or whatever you want to mute). Perhaps this is what you already meant, but it is not what I read (with my poor understanding of the English language).
You read that correctly. But we can already individually mute instruments in the mixer, which I guess is what I can also utilize. Although I would still prefer a master-mute button. Though maybe this is wholly unnecessary... :)

Sounds reasonable, but we'd definitely have to change the icon for the metronome toggle button :)