I always find this difficult to google for.. so just to save me time in the future:
extensions.conf: same = n,Set(__ATTENDED_TRANSFER_COMPLETE_SOUND=beep)
The __ at the beginning means the variable inherits multiple levels on calls or processes spawned by the dialplan.
For more information on ATTENTED_TRANSFER_COMPLETE_SOUND see:
Otherwise there is very other little documentation about the variable.