When creating a resource pack, you can namespace it by using this folder structure:
/<resource_pack_name>
/<resource_pack_name>/assets/<namespace_name>

if you want to add new sounds events in a custom namespace, you create a sounds.json file:
/<resource_pack_name>/assets/<namespace_name>/sounds.json

Now, the sound event is known by the game (see picture 1). However, the sound file has to be placed in the minecraft namespace in order to be played:
/<resource_pack_name>/assets/minecraft/sounds/test.ogg

More, if the sound file is not found, the /playsound command does not return an error (see picture 2).

Maybe related to MC-118737.