At the present time a single keyboard shortcut cannot be assigned to enable/disable specified A/V track. A user may achieve same by switching to the affected track (e. g. "2" for video track 2), then pressing Shift-H. While additional macro-keys mapping may be used to achieve the same I believe that for convenience it may be useful to have an option to define this in Kdenlive directly (e. g. Ctrl-(Alt-)-X where X stands for track number).