I have problems like these.
The thing I normally do is to use audacity to make it work again. First, open the music onto audacity by going to file, open and then select your song.
Then once you have opened the music, go to file and click export
once you have got that, chance the file type to "MP3 files" and save, then click "OK"
Now it should work on flash, hope this works!
But, make sure you download the "LAME" audacity plugin with audacity. If not you will not be able to export music.
links:
http://audacity.sourceforge.net/ -audacity download
http://audacity.sourceforge.net/help/faq_i18n?s=install&i=lame-mp3 - audacity LAME plugin download and instructions to install it onto audacity