3D Realms make available a pack with the full soundtrack as mp3 files. You can find the link at http://www.3drealms.com/stargunner/index.html, on the downloads section.
Stargunner is available now as freeware. As stated in the 3D Realms website: "You're welcome to enjoy playing the game, but this means that we retain copyright on the game, and you're not 'free to do what you want' with it. You can find the download link at the official page in http://www.3drealms.com/stargunner/index.html.
Stargunner was originally supposed to be an Amiga game with a PC conversion supposed to follow, but the PC version soon overtook and led development, and the Amiga version was eventually dropped. Most of the art was actually developed on an Amiga using Deluxe Paint.
Stargunner was the last game released under the Apogee label, before they committed everything to 3D Realms and their other divisions.