Custom resourcepack added sounds not playing

The bug

Since 17w15a custom resource pack sounds (not just changing vanilla sounds, the ones you have added and included in the sounds.json file) do not play a sound when /playsound(ed). This used to work fine in previous versions.

How to reproduce

1. Download the attached resource pack 17w15a Sound Bug.zip and load it
2. Use the following command to try playing a custom sound

   /playsound sound-bug master @a
   

   → The sound is not played; additionally an error message appears in the log indicating that the sound could not be played

Note: The provided resource pack modifies the nether quartz ore texture as well, therefore you can check if the resource pack was loaded correctly by looking at a nether quartz ore block.