If a resource pack includes custom sounds, they will not work if the resource pack is in a ZIP file, it only works when in a normal, regular folder.

Note that this bug was introduced with Minecraft 1.14, it worked before as I tested it in all MC versions back to 1.13.2 to know exactly when it started hapenning.

I included a simple resource pack, you can simply use it and test with /playsound minecraft:custom.intro master @a

If you then unzip the content and place it in a folder the sounds will work

Another way to test is to make a resource pack that replaces one of Minecraft's original sound, when trying to play the sound when using a ZIPped pack, it will play MC's original sound and when used in a folder it will play your pack's sound