audio delay is already in xbmc. *it just isn't assigned to any particular buttons.
you can change this by editing keymap.xml and assigning whatever buttons you want to handle it. *note that each button can only be assigned to a single action, though, so you might have to miss out on some other action. *(although, the left and right triggers are currently not doing anything when viewing movies, so they could be used).
the action id's for audio delay are defined in key.h as:
#define action_audio_delay_min * * * * * * * * * * * * *54
#define action_audio_delay_plus * * * * * * * * * * * * 55
thus, to map audio delay minus to the left trigger, you'd add:
<action>
* *<description>decrease audio delay</description>
* *<id>54</id>
* *<gamepad>lefttrigger</gamepad>
</action>
this needs to be added in the <window> </window> section for the fullscreen video.
enjoy.
oh, and subtitle delay is already there as well.