Add your music files to the CustomSoundtrack within your Metal Gear Solid 5 folder and these will be picked up in game.

Adding custom music is only possible in the PC release of Metal Gear Solid 5.

To add your custom music, copy your audio files to the CustomSoundtrack within your Metal Gear Solid 5 folder (defaults to C:\Programs (x86)\Steam\Steamapps\common\MGS_TPP) and these will be picked up in game.