The bottom line: If you're not getting sound from your rear speaker, the problem may be that the mute switch is malfunctioning. Try slowwwwwly switching it from off to on, with audio playing loudly enough to hear. You may find a "sweet spot" that allows sound to play. That's what happened to me.

The story:
I have an off-network Treo 650 and an on-network Treo 755p. I have Delorme Earthmate Street Atlas 2008 (HH) software on both devices. I sometimes use the 650 or navigation when I expect to need the phone or data network of my 755p.

Recently, I had problems with my gps software on the 650. Shortly after resolving them, we hit the road for a short trip. I noticed that there seemed to be no voice prompts this time (often there are too many). After confirming sound settings and preferences in multiple applications, as well making sure the mute switch was not engaged, I concluded that something must be wrong with the rear speaker. The headphone jack seemed to work just fine.

I figured "This must be the Treo 650 headphone jack problem so many folks have mentioned (or one of them anyway)." I decided to look up some repair information and start pricing replacement 650's (or just suck it up and use the only the 755p), when my girlfriend mentioned that she thought she'd briefly heard voices from the Treo, at some point, earlier.

I pulled the headset out and played something, but heard nothing. I tried moving the mute switch back and forth and that's when I noticed that sound was briefly coming out of the rear speaker between the "on" and "off" positions.

In the end, I set the switch to a point jussssst before the "on" position and there's been sound ever since. (I probably won't move the mute switch anymore, since it's probably on its way to the grave.)

Problem solved for $0!