With a few exceptions (e.g., hiding the OSX menu-bar with
ns-auto-hide-menu-bar), Emacs does not alter basic system settings generally controlled by OSX (e.g., keyboard shortcuts defined within System Preferences / Keyboard / Keyboard Shortcuts). The original poster has implied that Emacs is receiving the keyboard input of Command+ALT+-, which Emacs responds to by displaying a message
M-s-- is undefined. If the keyboard input had been hijacked by an OSX system keyboard shortcut for controlling iTunes, then Emacs would not have registered the input -- i.e., Emacs would have done nothing.
In a situation such as this, the original poster has two general options: (1) keep trying to define an OSX system keyboard shortcut that prevents Emacs from ever receiving the keystrokes [this is not done within Emacs, but rather at the OSX system level]; or, (2) write an applescript or other mechanism to control iTunes from with Emacs. [There are at least a couple of libraries already written for Emacs to control certain iTunes functions on OSX -- available by Googling.] Here is an example for opening Safari with an applescript from within Emacs:
(defun safari-activate ()
(let ((script (concat
"tell application \"Safari\"\n"
(start-process "safari-activate" nil "osascript" "-e" script)))
The keyboard shortcut at issue is a little tricky because ALT+COMMAND+- on OSX is actually creating an EN DASH (i.e.,
–, aka character code 8211) instead of a HYPHEN-MINUS (i.e.,
-, aka character code 45). The following example can be used to demonstrate whether an OSX system keyboard shortcut is intercepting the Emacs keyboard shortcut:
(global-set-key [(meta super ?–)] (lambda () (interactive) (message "Hello-world!")))
NOTE: In the above example, we use an EN DASH to define the keyboard shortcut -- not a HYPHEN-MINUS. If an OSX system keyboard shortcut to control iTunes had been in effect, then Emacs would be silent instead of generating the message Hello-world!.