Royalty free music

Most likely the reason is that they don't want there to be a problem for the channel. Since YouTube now monitors content all the time and checks for authorship music.

What possible reason could there be for them not to post my video? After all, they can just block the audio track, but there won't be any problems for the channel. This is already proven by many advertisers.

Maybe it worked before, as far as I know, now YouTube blocks not only the soundtrack, but also the video itself and you have to either delete the whole part of the video, or upload the clip again and insert other music. You can use a service with royalty free music. There's a huge amount of music there that you can use in your video. Youtube will not block the audio tracks. I'm telling you from my experience, because I often post different videos on YouTube myself.

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