You can associate a sound file with a dialog line already.
For example, for this line (in Loc/English/npcChat.trn)
GenericTalk1 "How can I help you, ##1##?"
if you wanted a sound file to player when that is displayed you could add a line like the following to the DialogSoundSystem entry in systems.gdb:
Depths of Peril, Kivi's Underworld, Din's Curse, Drox Operative, Zombasite, Din's Legacy, & Drox Operative 2