You should open an m3u file that you know works in notepad and one you know doesnt work in notepad. They're just text files. I'm willing to bet the one that doesnt work has some sort of comments in it.
Normal m3u files follow a specific convention of listing each mp3 filename in the playlist, one per line as such:
(01) [Es Vedra] La Hacienda.mp3
(02) [Glenn Morrison] Contact.mp3
But m3u format also supports embedding comments like
#M3U or #In Search Of Sunrise
Audiosurf doesn't seem to like the comments.
So if you open up an m3u file that doesnt work and you find it has comments, you can safely remove all the #'d lines and save it without breaking the m3u file and then it should work in audiosurf