I try to play an mp3 file. This works if I change the path to the file on my local webserver, but if I run this on an Android device the sound is not played and no error is
Did you try removing the /android_asset/www/ part in your code? Your app acts like a website and it thinks its root is the www folder itself.
/android_asset/www/
Edit: I'll edit comment. Code not showing somehow.