Hi All,
Just a quick report in hopes of helping someone else facing this problem:
Textmate binds Cmd-Opt-Ctrl DownArrow to scrollLineDown. I use this keystroke and it's counterpart for scrollLineUp frequently.
After upgrading from Snow Leopard to Mountain Lion I started getting a system alert sound (beep, pop, funk, as set in System Prefs) when using this keystroke and no scrollLineDown action in TextMate.
Long story short is that Xcode 4 eats that keystroke even when it's not running.
Fix is to open XCode, open Preferences > Key Bindings, and set the key binding for 'Jump to Previous Counterpart' to something other than Ctrl-Cmd DownArrow. This causes the inferred key binding for 'Jump to Previous Counterpart in Assistant Editor' to change from Cmd-Opt-Ctrl DownArrow to Opt NewKeystroke.
For example, setting 'Jump to Previous Counterpart' to 'F19' causes 'Jump to Previous Counterpart in Assistant Editor' to change from Cmd-Opt-Ctrl DownArrow to Opt F19.
I needed to reboot before it took effect, but that ended up doing the trick :)
Might also work in Lion, I'm not sure what version of Xcode that shipped with.
Hope that's a help to someone in future :)
Best, Steve