Simply copy all files in the rar into the sound folder of your Sins of a Solar Empire Rebellion directory (don't forget to back up your files should you wish to switch back).
Don't do this! Sins has a mod system for a reason! Never override original files, even if they're just music.
Instead, go to the "My Documents" folder, then "My Games". Navigate through "Ironclad Games", "Sins of a Solar Empire Rebellion", and in there you should see a folder called "Mods-Rebellion v1.01 (or current version)".
In that folder, make a new folder called "Homeworld Music Replacement Mod" or whatever you want to call it. In that folder make a new folder called "Sound" and extract the .rar in it (the music files should be directly in the Sound folder). Then go to the main menu, options, and mods tab. Enable your new mod and you have home world music, no need to override base files and you can toggle it off and on as you wish.
