It does already, but I would like to change them personally. One could replace the WAV file but I try not to override game files as much as possible.
This would need a script to register and catch the QUEST_COMPLETE event then playing the audio file. Problem is, I am used to World of Warcraft Lua programming, things are a bit different with Runes of Magic; otherwise I would be able to get it working myself.
There are also other reasons I need help building the addon for this, as it can be used for a few other ideas in my mind.