Yeah the BGM will not start when and
seemingly only when I load a save file with the script I posted above. (Pic of it in an event below.)
If I load the game's save file with the default built in save window everything works just fine. (Pic of the menu I mean below, I'm currently using Yanfly's Save Core for the window appearance, but as far as I can tell I think the methods of saving/loading the actual files are the same.)
The first half of this issue doesn't seem to be entirely related to your plug in (I used a blank project for reference.) If I only did what I listed above in this post, the audio would NOT resume in a blank project, the fix to this is assigning the current BGM data to a variable, like this:
And then when loading a file via script, check that variable to properly resume what was the currently playing music at the time of saving (circled in red below):
If you have any other questions I'll be happy to provide as much as I can!