Singleton working in editor, but not in build

Howdy friends,

I've been working on a game for college recently, and decided to implement a singleton to control the music throughout. In the editor, it works just fine with the right music playing at the right time. When I build and run, however, it always seems to be one track behind e.g on the main menu it plays the splash screen theme, then if you went into the settings menu the main menu theme would play, then going back to the main menu would have the settings music played.

Here's a link to my MusicPlayer script if needed

I have a feeling it could be to do with Unity's own splash screen confusing things, as this is the only real difference I can see between the build version and the editor, but this has had me stumped for a while now.

Any help would be very much appreciated! Let me know if there's anything else I can provide to help you help me. :D

Cheers.