If the m3u uses absolute paths they will be wrong on your device. For example, if the song is stored on your computer as:

/home/etuoyo/Documents/Music/airguitar.ogg

and on your device as:

/home/user/music/airguitar.ogg

then all the files listed in the m3u will point to the wrong location. You can easily check this by opening the file in a text editor.

I don't really know whether m3u's are supposed to use relative or absolute paths (relative would be ./airguitar.ogg if the m3u is in the same place as the file), but you could at least fix the problem with the following shell command:

sed 's,/home/etuoyo/Documents/Music,/home/user/music,' old.m3u > fixed.m3u