I know how to play custom music in NSMB 2, but my question stands in the adding of additional tracks.
With tilesets you add them to the unit folder; can the same be done by adding files to the sounds folder?
Is code hacking a necessity under the assumption it's possible in the first place?

Yeah just adding the files to the folder isn't enough. You also cannot add custom entries to the sound archive to reference the added tracks. The game has a hardcoded list that connects the music ids in the level to music ids that are played from the sound archive which then references the stream files.

So yeah, without code mods this isn't doable.
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