Quote from our review regarding this:
"Want to listen to MP3s? The Treo supports stereo output through headphones but sadly comes with only a mono earbud headset with mic. We tried the 2.5mm stereo headphones included with the Audiovox SMT5600 MS Smartphone and they worked fine for phone calls and stereo MP3 playback. PalmOne sells an optional adapter so you can use stereo headphones with the Treo for music and video playback. Sound quality is good for MP3 playback using the included RealPlayer, though not the best we've heard on a PDA."
The full review is here: http://www.mobiletechreview.com/Treo_650.htm