I submitted a bug report similar to this recently & it was marked not a bug & working fine, but I don't think I explained myself clearly enough, & so after a couple of days of trying different things that I could be doing wrong Personally, I've yet to find a way this works. If you're to dismiss this bug, please, don't do so hastily, as I might be miscommunicating something somewhere, or alternatively you could be misunderstanding something.

When in 1.13, using /playsound test master @a ~ ~ ~ 1 1 1 with a custom sound in your resource pack (explicitly in your resource packs folder, not in the world folder itself, I've not tested for that) named "test", nothing seems to happen.

When in 1.12.2, this command works fine. There doesn't appear to be a change in syntax, nor any reason the file would refuse to play in 1.13. I've tried isolating it, even, to a single resource pack with Only the custom sounds, which works in 1.12.2, but not in 1.13.

I've attempted to nest the sound within a subfolder, within predirected subfolders like entities, and even used every namespace file I can think of; none of which seems to work. However, when a sound is replacing a different sound, it seems to work fine. The issue seems to only be with custom sounds.