@Nasko To re-encode it use the Audacity editor I linked in the previous post, you simply have to open the sound file and then go to File > Export > Export as MP3
Try this guide for that https://manual.audacityteam.org/man/mp3_export_options.html
On the other hand, it’s also possible Windows 10 is blocking Pencil2D components during video export. If possible please take a look at this guide to lessen the instances where Windows10 will totally mess up your work: