HeadCold
HeadCold is a small PalmOS utility that will allow you to direct the sound output of a Treo600 or a Treo650 to either the built-in speaker or the headphone jack. You can also chose the auto mode, which will automatically switch the output to the headphone jack whenever a set of headphones is plugged in. Unplugging the headphones in auto mode returns the sound output back to the Treo speaker.
HeadCold is FREE software, released under the GPL license. You may download both the binary and the source code free of charge, and modify it if you wish, as long as you adhere to the terms outlined in the license.
Download HeadCold 1.0a Palm OS Binary
Download HeadCold 1.0a Source Code
The project page can be found here. On the project page you can find more information, public forums and feature request forms.