Hi, I just created an account just to share my experience with you!
Yesterday I bought an FM Transmitter for my car so i can listen to the music on my Lumia through the car speakers. It worked great! I used it one, ive had my Lumia 920 for 2 days and now i couldnt hear calls or music or anything that required Sound output. Strangely enough Ringtones and notifications worked fine. Keyboard sound didnt work, nor was i able to use the Voice Recognition App.
I tried MANY things, cleaned out the jackport using a q-tip (incase it was jacked), tried searching online for a possible fix, NOTHING came up. I even rebooted 4/5 times and nothing.
But then I had one last option left. An option i was trying to avoid. I had to completely and utterl reset the phone back to its factory settings, everything on my phone was deleted and reset as if brand new. After the Initial Install; ALAS! SOUND IS BACK.
So try to do a hard reset/factory reset. I am 99% certain that will fix the problem. - Go to Settings>About>RESET YOUR PHONE.
To those that haven't had it yet, take this precautionary measure;
>While your phone is working perfectly, BACK IT UP. So when this problem happens you can easily RESTORE to that backup and it should work fine (a factory RESET is still necessary).
Hope this helps anyone
And Good Luck.