This may help?
Problem on my lumia 520, I purchased in January 2014,started about a week ago. All of a sudden(!) I could not hear the caller?s voice through the internal speaker, and the screen went blank so I could not see to press ?end call?. The callers, however, could hear my screamings OK! I found out that pressing the ?speaker? when initially replying a call made loudspeaker work and from then on I replied all calls on open conversation although very inconvenient and public!
I assumed it was a hardware problem and a complete disassembly and re-assembly did not fix the problem. So, I started shopping around for a replacement.
Then someone here, on the lumia 920 and 928 board, gave a solution which I followed. Goto, Settings, Audio (enhancements) wait for loading. There is a ?flip to silence? toggle switch that controls the ?flip face down to silence the ringer?. Turn it OFF.
I applied it on my 520, switched OFF and restarted my phone. It resolved the problem and I could hear the callers again.
A few hours later, with mobile in pocket, the problem came back and the caller?s voice was extremely faint (the speaker phone is working OK though but it is public!). Again I switched OFF and restarted my phone. It resolved the problem and I could hear the callers again loud and clear.
I believe it is a combined hardware and software problem with a certain tilt switch operating erratically.
The phone otherwise, works OK and I like it.
It will be nice to hear of a final fix.