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.
Matthew Chan
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.