From Toysoft Development:
The ultimate solution to your Treo 600/650 headphone problems: Freedom. With Freedom, you can finally listen to mp3's and audio with any headphone and no longer have to deal with the speaker phone.
Freedom detects whether or not there is a headset, and plays your sounds through the headset if it's plugged in, or the Treo speaker if there's no headset available. Freedom works with the Treo/PalmOne supplied headphone and all other headphones that is compatible with the Treo.