shortcut button best practices for 508 compliance

Regarding setting shortcut buttons for captivate and 508 users:

I've been searching for shortcut button best practices for 508 compliance and would like a list of which buttons would be best. For example.

Play = P; closed caption = C; Next = N; Back = B; Exit = X, etc. I do not know what would be best for pause, perhaps S for stop.

What have others been doing?

Many thanks, C