YouTube, and all platforms for that matter, normalize all audio to the same volume (I think it's -14 lufs), which is why music played at a higher volume sounds quieter. I learned that the hard way. Hehe.
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Yes, I consider that when uploadings my songs. After checking around what happened is I used OGG files instead of WAV for the video and that cause more compressed audio when uploading it. Even Youtube recommend using WAV or FLAC. I didn't thought it would affect that much, but well I learn something new :D