I can’t figure out a pattern. The are albums where you can lower the volume on the vocals on some songs but not others. I’m guessing the main reasons for songs being incompatible are:
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The machine learning algorithm they’re using can’t isolate the vocal track on that particular song
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The song doesn’t have lyrics on Apple Music (how do those get populated? Do they have a human go through every song and confirm the lyrics and time the little highlight thing to line up? Maybe they’re not finished with all the songs in the library?)
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The record label won’t allow Apple to use the Sing feature for that song
I would assume more songs will be Sing-compatible over time, but I’m curious is anyone knows how this works.