Share your experience!
If a software repair didn't help,you should stop by a sony service.
They have excluded any hardware problem?
Wait until some others post then.
I experienced this phenomenon about three times now. It got me puzzled.
I found out that connecting and disconnecting headphones solved the issue.
It looks like the phone did not realise my headphones was disconnected. Therefore there was no audio output to the speaker.
You might try connecting and than disconnecting the headphones. If this does not work directly you can reboot your phone and then perform the connecting/disconnecting.
It seems that sometimes the phone stays in headphone mode or vice versa.Sometimes,disconnecting the headset while listening to music or while in a call sorts it out.