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.
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I have encountered this problem as well. I am currently developing my own Minecraft subway system which is heavily using custom sounds via resource packs for announcements, they worked fine from 1.7 through 1.12.2 in same syntax (similar to the description above) and when I start testing them in 1.13, they weren't working as well. The json is valid and all sound files in .ogg and names in lowercase.
& my thorough apologies if the issue ends up being solely on my end, but for risk of it Not being on my end & being an actual bug that was missed on the fact alone that I didn't explain myself clearly enough, I think it's worth a second glance.