I had my Pre wired to my desktop speakers watching a DVD. When I unplugged the speakers I had no sound using any of the audio or video apps. Ringtones and Text message notifiations worked fine. I went to Palm's Forum and found this:
Was sound working correctly through your speakers with your Palm device and suddenly disappeared? Does the sound work correctly using wired headphones? This thread may be a solution.
In many Palm devices, there is a tiny microswitch inside the barrel of the headphone jack. Over time, it can become stuck due to a combination of dirt, finger oil, and homeless dust bunnies trying to find a place to stay! 
Freeing this switch returns normal operation of the speakers. Sometimes the action of inserting and removing the plug from a set of earphones will free it; other times, it may be a bit more grimy and can fight back.
Many users have found that placing a tiny drop of an electronics-type lubricant (WD-40, contact cleaner, etc.) on the plug and inserting and removing several times can bring success.
Note: This is not an official sanctioned repair from Palm Inc. - I'm a volunteer here, and have seen this work in many situations.
Hope this helped,
WyreNut
I have this issue with my kids' Centros and was hoping not to have it on my Pre.