Can the average human train their voice to be capable of singing well enough to be ‘decent’? (not pro)

Yes. And moreover more people than not have perfect pitch when singing from memory https://news.ucsc.edu/2024/08/earworms-music-memory-perfect-pitch.html

The Farcaster formerly known as Kchamp. Books, beats, bodybuilding, bright ideas and beautiful things.

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Wow, there’s a scientific backing to this ?
I actually do feel, whatever your brain iterates often, you get better at, so your brain singing and replaying the song in its melody, makes you a better singer