Method 1 - Replace the sample music.

The easiest method to add a new soundtrack while in Offline FinaleFireworks, is to replace the included sample mp3 files with your own.

The downside is there are only 3 files that can be used. (Although you could reuse them over and over again.)

Here's a view of the 3 sample files in FinaleFireworks:

The directory could vary slightly, the directory under "Users" will likely be your full name. I believe AppData is a hidden folder, so be sure to enable view hidden folders to get into it.

This is File Explorer. You should be able to just copy your mp3 files into this directory via drag/drop or cut/paste.

In my case, I renamed the original "2min_1812.mp3" to "2min_1812_ORIGINAL.mp3" and then renamed my new soundtrack "2min_1812.mp3".

Start FinaleFireworks, create a new show and add "1812 Overture" as the music and you should instead load/hear your soundtrack.

The same can be done for the 2min_extreme.mp3 and 2min_wagner.mp3.

I'm not sure if it's required, but it's probably best to update the files while FinaleFireworks is not running and then start it when done.