Alert Sounds Twitch?

What do I have to pay attention to? What kind of music can I use?

You can have Copyfree play music.

But there are also streamers who do not use copyright-free music. Did they then buy the licenses?

Either bought it or they don't care and risk a ban.

There are three options, at least:

The owner of the rights to a title has not yet noticed the act.
He has no interest in prosecuting the crime.
He granted the perpetrator a UrhG https://www.gesetze-im-internet.de/.../__31.html, for example through gema.de.