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.

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.

More information: https://headcold.sourceforge.net/