There are limited time event sound packs that Discord makes available (and default) during various seasons.
After the time period has finished, the sound packs are no longer selectable and for all intents and purposes unobtainable to end users.
I think this is a shame so I have exerted some small effort to figure out how these packs are looked up so I can archive their contents. Fortunately, at least for the time being, it appears Discord keeps the old sound data on their servers. But it might not always be the case.
Within the client bundle (viewable within the client or archived consistently at Discord-Datamining) there are a number of lookup tables.
Because of the nature of webpack (and due to the size of the file) I can't link you to where they are specifically since they're subject to change, but I can show you what they look like so you can find them yourself in the future.