This is what I found so far.
Open the ISO with UMDGen 4.00. Music files are usually in AT3 format, sometimes it will be in a different format, it depends on the game. The files are normally found in a folder called "Music" / "Audio" / "Sound", etc.
You might have to look around in the ISO if the game has a lot of files.
So once I do that, Now I have to figure out how to convert it to MP4.