In my case, the video player, MP3 player, and Pandora don't have sound, but everything else does, both on headphones and on speaker. So I can hear the "swipe" sound when dismissing a card, both on speaker and headphones, and the ringer works fine on both as well. but NO multimedia apps will output sound to the speakers, JUST the headphones!
I've tried a soft reset (orange, sym, r) and a power down and back up, and I've even pulled the battery. None of those worked.
I tried the headphone jack in and out, no go, and I blew into the headphone jack. no go.
I'm out of ideas and frustrated. It was working earlier today too.
I tried popping the battery out WHILE THE PRE WAS ON and it fixed the issue. So it's absolutely a software problem. Doing a full shutdown SAVES any software settings (such as being in headphone mode) immediately to non-volatile memory, thus the problem is back when you restart. but popping the battery rather than doing a full shutdown would dump the volatile memory IMMEDIATELY, thus wiping any settings.
So, at least in my case, the problem was software not hardware.