Note: This might be WAI.

The bug

Sound file names referenced by sound entry in the sounds.json file do not need the prefix minecraft. However, if a resource pack contains a sounds.json in an external folder (for example custom), then all sound file names in this sounds.json file need the prefix custom:, otherwise they use minecraft: as prefix. For plugins this might be pretty annoying and inconvenient because they will very likely refer to many external sound files.

Example

Resource pack structure:

• pack.mcmeta
• assets
  • minecraft
    • sounds.json

      {
          "testfolder.testsound": {
              "category": "ambient",
              "sounds": [
                  "testfolder/testsound"
              ]
          }
      }
      

  • custom
    • sounds.json

      {
          "testfolder.testsound": {
              "category": "ambient",
              "sounds": [
                  "testfolder/testsound"
              ]
          }
      }
      

The bug is that the sound of custom:testfolder.testsound is loaded as minecraft:testfolder/testsound despite being in the custom folder. You need to add custom: as prefix to prevent that.

How to reproduce

1. Load the attached resource pack (External sound test.zip)
2. Use the following command (does not use prefix)

   /playsound custom:testfolder.testsound ambient @p
   

   → The sound won't play because it is loaded with the minecraft prefix

3. Use the following command

   /playsound custom:testfolder.testsound.prefix ambient @p
   

   → The sound will play because the prefix is provided

The reason

The following is based on a decompiled version of Minecraft 1.10 using MCP 9.30.

This happens because the method net.minecraft.client.audio.SoundListSerializer.deserializeSound(JsonObject) creates a Sound object with the given name, which uses the minecraft: prefix if no other prefix is specified.