Satire:
Ahh, yes, I see that you are trying to use the
Sound - Play Music
trigger action. If you wish to use this action, the way to do so is to buy a CD of the Frozen Throne video game from a reseller online. Insert the CD into your computer, click on Install, and you will get a warning that you need the Reign of Chaos client. So order a CD of the Reign of Chaos as well. Then, insert that CD first, perform the install step, then remove the Reign of Chaos CD from your disk drive, insert the Frozen Throne CD, perform the install, and then launch the World Editor program on your Windows 11 computer using the application list present in the start menu. Inside of the World Editor, create a new map. Choose the Trigger Editor from the toolbar, then create a trigger. Inside this trigger, click on "Sound - Play Music" as the action. For the music, choose an mp3 file that you choose in the Sound Editor. You will see that the music you have chosen plays at the time when the action fires.
Reality:
On Reforged, the function to play music likes to sometimes play the music of the Reforged instead of what you asked. Last time I checked, you could basically work around the problem by using all the "Stop music" and "Stop thematic music" type functions, all at the same time, and then after stopping everything, play your custom music, then maybe stop it and play it a few times, and you can sort of get into a state where you coerce the system into doing what you want.