As the title says. I have no problem with music in main menu, during briefing scenes and even when I played a tutorial. But when mission/s start I hear soundtrack for maybe a 5 seconds and then mothing. Unit voices and environmental sounds are OK.
I already tried to reinstall the game (twice) and launch as administrator and in Windows XP/98 compatibility mode and I still have this problem.
My OS is Windows 7 x64, sound card is Realtek ALC-892 and my game is a GOG version.

Same here, at first I thought it was supposed to be like this, but makes no sense considering theres a slider for music in the option menu.

I had the same problem. I presume most people automatically use the hardware render option when starting the game for the first time and because of some glitch or bug the in-game music doesn't play?

The solution is to select the software render option instead. Start a mission and the music should play. Exit the game and restart by selecting the hardware render option (Choose your Graphics Card), then start a mission and just like magic the music should now work? Weird, but it worked for me. :)

Add -render to your shortcut to get the render options to show.

Looks like I jumped the gun, as the music stops after you get to mission 3. I'll look around for a fix and post it here if I find one.