I experienced this. There are lots of posts around but no answer.
The most common problem is not with the software.
The speaker has is connected to the phone by 2 tiny metal wires. Sometimes they compress too much and loose contact. You'll find the headset still works, but no rear speaker.
You need to take the phone apart carefully (its easy if you follow the disassembly photos in the sticky thread). Then slightly straighten the wires on the speaker, then put back togeather.
Easy peasy. If you need a photo of the speaker then email me and ill do my best.