I had this happen to me recently and found out whats wrong the phones kinda slow and thinks the headphones are still plugged in and if you take out the battery and put it back in it saves that you had somthing plugged in and dosent help. For some people they can fix this as easily as plugging the headset back in and pulling it out slowly but for me this didnt work so what i did is, i turned on pocket tunes and you souldnt hear anything and make sure your sound is turned on, then take a small pin and stick it in the headphone jack and wiggle it around untill you hear somthing and then quickley pull out the pin.
Hope this helps

The pin fix with Ptunes did the trick for me on my Centro after trying everything else.

Yes, the pin trick does work. The palm centro does have a faulty headset jack. By the way, if you want to get a new palm centro for free, listen to this. I had the same problem that you guys had last summer. I took the phone to the verizon store and told them what was going on. They replaced it for free (almost no questions asked). If you want, you could plug a headset into your phone, take it out, and make sure your phone does not make any noise. Go to the verizon store and tell them the problem. They will probably replace it for free. If not, just go home and fix it with a pin.

I didn't need some kind of pin, the headphone plug did the work for me, while playing pTunes.