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
Did you put the attached zip folder directly into your resourcepacks folder? Or did you take out the folder inside?
[media]If you placed the zip folder into your resourcepacks folder, than it is not working because of the way you have it set up. Inside of your zipped folder, you should not have another folder named pack. Instead you should have it go directly to the pack.mcmeta and assets folders.
Please see the attached resource pack and see if it works for you.