No audio when exporting sound.

Previously, I was able to export audio parts. Yesterday when I did that, only 3 files had sound. Now when I export audio parts, there is no sound at all!

I installed Musescore 4.5.2, which I use. I downloaded with original score from musescore website. I emailed to my friend to export audio and he didn't get would with, but we get sound from one of his files.

I don't know what to do. I have musicians waiting for their audio parts. Please help.

Sonya