A question of using music.

[AxalaraFlame]

Can I simply adapt some classical music or technos from a famous movie or I have to pay them for copy rights before using them for a campaign in wesnoth?

[Iris]

1. No, you cannot, assuming you meant “music from a famous movie that is in classical style”. Otherwise things get more complicated and the answer will vary depending on the original composer, whether there’s any valid copyright holders at this time (heritage, etc.), and any specific IP regulations in the country where the work was originally created.
2. Paying them for using it would not be enough. You’d have to pay them to relicense it, which would be near impossible in most cases since the GNU GPL jeopardizes their ability to profit from licensing their work. You’d either be required to pay them a ridiculously large amount of money, or they would refuse outright.

[AxalaraFlame]

Then, how can I use them properly, if I want to use them?

[ancestral]

For personal use only, perhaps. On your own computer, not distributable.

[West]

classical music

Classical music is normally in the public domain and free for anyone to use and adapt. However, RECORDINGS of classical music are usually copyrighted so you would need to record (parts of) a piece yourself based on the sheet music.

Edit: Right, I see now that by "classical music" maybe you mean "orchestral music" in general. What I wrote above only applies to music by the classical composers e.g. Mozart, Bach, Beethoven etc.