Latest nightly sounds ok (rules out the versions before the new mod system).
And to answer your question (equally valid to my ones
), my SOUND folder does only contain the originals.
We could always compare folder contents .. although i have to check my modstructure here (i might have broken some things after moving from cloud storage provider) .. so that will somewhere later this week.
In the meanwhile , could you check if it works when you place your music in a subfolder of your mods named SOUND (eg .../mod/mySuperDuperMusic/SOUNDS ), i thought that did the trick for me.