I would've said bitrate, sample rate and EQ will all make a difference. I've come across a few phones that only sound good when you throw away one channel instead of mono'ing them. Obviously some weird phasing things going on that get cancelled out on a mono speaker. My N95 is 'stereo', there's even a mode that messes with the phase so it sounds '3D'.

Can you download the existing ringtones and listen to it on something decent? I would wager they have their mid's boosted.