Do you any equaliser settings or sound enhancement set in iTunes? If so, try removing them. I should think iPhoto probably doesn't take notice of these settings when it plays back tunes.
Went in under prefs/"playcolumn" and disabled (own translation as i have swe. vers): Play with overtones, Audioenhancer and Volumecontrol. There was certanly a change in audioquality. To the better that is. Still not as good as iPhoto though. Will do diffrent "enable-disable" combinations with above pref. adjusters. Maybe "Play with overtones" should be enabled (or not).