*I do not recommend using this with any other programs, especially online multiplayer games*

Due to how the program was coded when the window loses focus the music and audio stops. The best way I've found to fix that is via a program called Special K, its a DLL injector that lets you change different aspects of the program. Once you have it install you launch Descent through it and use hot keys to change settings. Find and check the box I show in the picture below.

https://i.imgur.com/HzpeckF.png

https://wiki.special-k.info/en/SpecialK/Global#installation