The mute system is a little confusing. Basically, Twitch auto scans the video for what it thinks might be copyrighted audio, then it mutes that section for you to prevent you from getting a copyright strike. This is all automatic and set to extremely paranoid because when a big wave of copyright enforcement went around a few years ago, that was equally paranoid and ridiculous. People got copyright strikes for 2 second snippets of something you could barely hear in the background. It was bad.

But the auto mute isn’t a strike or even a warning. It’s no authority at all. It’s just a random automated guess at what companies might be looking for when doing their scans. It’s there to protect you in its weird and uncomfortable way.

You can dispute, but that puts the responsibility for any claims on you. You’re waiving the protection. It might be fine, but if it isn’t you suffer the consequences. So generally it’s usually not worth the risk to bother. But a mute is no big deal. It doesn’t hurt you or count against you in any way.

tl;dr: nobody is actually claiming that audio is copyrighted, except some robot scan thought it might be and better safe than sorry.