Resourcepacks can only be used to replace sounds, and not add them. Or in other words, the playsound command only loads sounds from the minecraft resources folder, and should be loading a new list based on the resourcepack that is loaded.
Playsound should support custom sounds, and not just overwritten sounds. Allowing custom content without diminishing the original game's content.
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Perhaps, but it is unlikely you would find one here, as this website is used specifically to track bugs.
They set it to resolved because it "works as intended" and we'll never get any answer besides that, might as well give up now. Never going to get anything from these people so we just have to deal with the horribly implemented and crappy design of the feature that could be easily fixed.
The reason why it is listed as such is because, currently, the resource pack system is functioning the way it is intended. It is not presently made to function with extra sound files. That is not to say that this will not be implemented in the future, but as this is strictly a bug tracker, we aren't currently tracking features to be added.
Duplicate of MC-31762, please use the search function to see if your bug has already been submitted. Currently over 43% of tickets are being resolved as duplicate.