Hi Julia.

Sorry to hear about your dad.

Noel is very knowledgeable guy, and there are many others who will help you if things get tricky.
I am average user, let me try to look at this from a "general" user perspective.

When you open SGU file and press PLAY button, can you hear the sound? If yes, that is already most that you will need to export MP3

If you can hear the sounds, follow these steps:
Go to menu on top of the screen called "audio" under that, locate "Export Song as Audio File", Click that. Menu box would pop up. Under "Format" choose MP3 and uncheck "one file per track" if that box is checked. Click large "Render" button on the bottom. It will show you where it will save MP3 file and the name of it.

I hope it helps. If not, please come back to this thread. Stay strong.

Misha.