Opening a local .mp3 file with AppleScript Editor
Assuming that by "open" you mean play the mp3, the following script should do exactly what you want:

set myFolder to POSIX path of (path to me)
set contentFile to myFolder &amp; "Contents/Resources/Jingle bells.mp3"
do shell script "afplay " &amp; quoted form of contentFile


Above, we set myFolder to the POSIX path of .app file that is being run. Then we set contentFile to the POSIX path of the .app file plus the location in the internal directory which links to the file you want to play (Contents/Resources/Jingle bells.mp3 in your case). Finally, we use the shell command afplay (man page here) to play your mp3 file.

Hope this helps!

Best, Tom