Just tried in CS6 and CC and works as expected (Windows).
This also works for me in CC (Windows 7). Are there any other steps that you did?
Just what I described.
The 'set default' does not 'take'.
On Mac 10.7.5.
Hmmm, just tried this on Mac 10.7.5 and it still works for me. Changed the default audio transition to each of the three and Apply Audio Transition applied the correct transition each time.
Not working for me. Just tried again, after a restart.
The 'constant gain' gets the orange rectangle, but it still applies 'constant power' every time.
OK, so the issue is that I've been using the 'Apply Audio Crossfade Transition' command, NOT the 'Apply Default Transitions to Selection' command.
Which is still rather annoying, since it functions differently ... seems you have to select the transition with the mouse, whereas the Crossfade command automatically applies to current position.
Can't you change the Crossfade command to be 'constant gain' ???
update: OK, I guess I can use the 'Apply Audio Transition' ... so what's the Apply Audio Crossfade Transition for?
Aha. There is a difference as 'Apply Audio Crossfade Trasition' (as well as Video) are basically hardcoded to Constant Power (and Cross Dissolve for video). It's been this way as far as I can recall (went back to CS5.5 to confirm). The 'Set Selected as Default Transition' is designed to work with 'Apply Audio Transition' (and Video).
If you look below these option in the Keyboard Shortcuts, you'll see other specific options like 'Apply Video Dip to White Transition', that give you an idea that assigning shortcuts to these Timeline panel functions are specific. Granted, the naming for 'Apply Audio Crossfade Trasition' could be more specificly named to avoid confusion.
All that said, please feel free to submit a feature request/bug report. https://www.adobe.com/cfusion/mmform/index.cfm?name=wishform