This is the problem I'm having. I just got Audacity, and checked myself: both the original file, and the copy made in the resource file are 44.1kH (Opened them both, bottom left hand corner, already says 44100 - i'd already checked them with VLC as well, and the friend who made the music for me says he definitely did them as 44100 MP3s. Which they are.)
However, when I try to export, it's saying that it's 32000Hz (as seen in the screenshot I posted) - and then subsequently doesnt export the flash file.
I'm going to try with a different file, and see if the same problem occurs.
Update: Made a blank game file, and tried each of the 4 pieces of music my friend sent me. They are all showing as 44100 MP3s. However one of them doesnt work. The other three are fine. Now we've pinpointed that, he's looking into if he did anything differently, and one we have some answers I'll post them here and hopefully others can avoid the same problem!
Upadate 2: Ok. So. Basically. Changed the filename. Now it works. I don't even... well I guess my issue is solved anyway. Thanks for the help peeps.