If installation through Mod Manager doesn't work, here is
how to manually install, it's not difficult! 

1. Enter the Fallout NV files (Usually SteamLibrary\steamapps\common\Fallout New Vegas)

2. Go to >Data
          >Music 
           >Special 

3. Copy the MainTitle.mp3 file in the rar, and overwrite the original with it.

4. Close, and boot the game up

5. Enjoy, and Begin Again!