Extension's Shortcuts

Doesn't triggers if you run them inside of the pop out window.

You can change the shortcut from chrome://extensions/shortcuts

Shortcuts in Popup Window

You can't change them, Nor run them outside of the window.

Description	Keys
Play or Pause	[SPACE]
Fav/Unfav the Track	L
Increase Volume	[SHIFT] + [ARROW UP]
Decrease Volume	[SHIFT] + [ARROW DOWN]
Mute Volume	M
Repeat (One, All, None)	[SHIFT] + L
Toggle Shuffle	[SHIFT] + S
Previous Track	[SHIFT] + [ARROW LEFT]
Next Track	[SHIFT] + [ARROW RIGHT]
Seek Backward	[ARROW LEFT]
Seek Forward	[ARROW RIGHT]
Open SoundCloud Tab	Q

Note

Those two are different thing. While the first one runs everywhere inside/outside (optional) of chrome window, Second one only runs inside of popup window.