The DF Mini Player IC can be used to play mp3 audio files when triggered by the Microbit.
Using Audacity, or online alternatives such as Twisted Wave, sound snippets can be stored as mp3 files on a micro SD card, which is then inserted into the DF Mini Player.
Files should be named 0001.mp3, 0002.mp3, 0003.mp3, etc.
FreeSound: You’ll find a lot of atmospheric music in FreeSound that would work well as a film score. They also have a large amount of sound effects if you’re looking for those.
CCMixter: This is a pretty awesome resource for Creative Commons songs.