I dont own a j720 but my Nec MP900 had upper average sound. To test the audio quality i would suggest you install TCPMP and throw some audio files on a compact flash card and see what it can do by listening to it plugged in to some amplifier or something.
you should get TCPMP and i think you can download it by going to the downloads section and active syncing or putting the cabs on a CF card and going from there.