Whenever I run Marathon, the in-game music never seems to play. (This applies to Durandal and Infinity too, for several of the background sounds)

I tried installing QuickTime 4, but that didn't work. I have 'Background Music' ticked in Marathon's prefrences, so don't see why it shouldn't be playing.

Any suggestions?

You tried installing Quicktime 4 , but it didn't work .

Which didn't work? It didn't solve the problem , or the installation failed?
You might also want to check the preferences if the Background Music dial is set to zero .
In that case , even if you enable it , it would only be zero .

Also , check your system sounds . First , check if the Windows sounds are working . Then , run Basilisk II and find any other sound you can play in it . Last thing I would do is check the volume mixer in Windows while Basilisk II is running , in case the dial is also set to zero .

If not , check the Marathon folder if there is any track for the background music . There is a possibility where the music is not included with the game (probably removed by the uploader to save space , or a disk error) (same thing happened to me in Red Alert 2) . In that case , well , if you tried all the methods I recommended , then there's not much I can do to help .

Here's a review :
(1) Check Windows volume .
(2) Check Basilisk II volume .
(3) Check the Volume Mixer .
(4) Check if there is a track or anything related to music in the game folder .
(5) Check if the game packaged was original (as in , all files remain as how you would buy it in the store , no rearranged or modified files) .

Good luck .