In the config file (the one in [user]\Documents\My Games\D4 Dark Dreams Don't Die DEMO\Ms01Game\Config) and then launching the game executable (Win64/D4Game.exe) directly.
Reminder that
1) D4's soundtrack is so spectacularly oddball and awesome.
2) Four tracks, including Salvation, Theme of D4 and Drank Dead DOM Die are available as a mini-album purchase. The studio, soundtrack versions.