DBM now supports user added sounds for more experienced users. There is no GUI for this feature as it's a more complex feature and having a GUI for it could confuse new users that may enable it without adding sound, which would then register blank sounds with libsharedmedia (LSM).
Right now, you must edit your own saved variables on a per character level to set number of installed custom sounds from 0 to whatever the appropriate number is. It will then register media which it will ASSUME is at following location:
Interface/Addons/DBM-CustomSounds/CustomX.ogg where X is numbered sound. This supports 0-9 only.
Sound files MUST be ogg format. On Mac OS X I use X Lossless Decoder (XLD) which is free sound converting app.
Note: This is requires 5.4.16 or later.