Method 2 - Replace some of your old music files

All the previous soundtracks that you have used with FinaleFireworks on your computer are saved on your computer.  They get assigned a unique numeric filename and lose the .mp3 or .wav extension.  So "July 4th, 2020.mp3" might get renamed "1661492".

Navigate to the following directory:

C:\Users\YOUR USERNAME\AppData\Roaming\FINALE Fireworks\sound\user\api\usermedia\sound\YOUR FINALE ID\item

There will likely only be one finale ID under the sound dir. Each user is assigned a unique finale ID. It's likely a 7 digit number.

Here's what mine looks like:

Those files are soundtracks. You can copy them elsewhere adding back the .mp3 extension to determine which soundtrack they are. If you know the show they were used in and have the HBS file for that show file, you could replace them with a new soundtrack and repurpose the HBS show.

For example, say file "166147" was a New Years soundtrack and you have the HBS file for that show. You could replace the 166147 file with your new July 4th soundtrack, load the old New years HBS file and you should hear your new music. You could clear the old show elements and create a new show with a new soundtrack.  

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

This method is probably the hardest since you have to track down the sound and show files yourself.