Copyright laws about music from game demos?

The demo would have a license you agree to when installing. You'd need to read that on a case-by-case basis.

Generally speaking, the demo likely just gives you permission to use the whole thing together, and not to take it apart, etc. And no, you would not have any "rights" to the music files just because you installed some game demo. But again, you'd need to read the specific license of any given demo. Some may say something different.

This falls under fair use.

As long as you are not using the music for public purposes, for a fee or not,

there is nothing wrong with you keeping the music files to a game on your computer.

Copyright laws prevent you for making a profit from the music. As long as you don't do that you are OK.

No

You have no right to use music which was lent to you for temporary use. You may "hold" this music, as there is no requirement that you remove such things from your hard drive, but you can not use this music as if you owned it.

No you do not have any rights to them other than for personal use.