These music files are part of The Mana World Project.
(c) 2006-2014 The Mana World Development Team Released under the same terms as The Mana World, the GNU GPL. Of which you should have received a copy via downloading this package.
The recommended way of getting music in the ManaPlus client to work in The Mana World is to check the "Download music" checkbox under the "Audio" tab of the Manaplus client settings.
Check the music-license file for a list.
In addition, "Ride of the Valkyries" is a public domain recording of a public domain song, taken from here: Ride of the Valkyries.ogg.