Troubles with resetting PRAM I followed Apple's instruction to reset PRAM ("Press and hold the Command-Option-P-R keys before the gray screen appears.") and the only thing that happened was that my Mac rebooted repeatedly until I stopped holding the short-cut. After releasing the keyboard OS was loaded but without any startup sound and my original problem remained (see Original problem)

I found out that not being able to reset PRAM may be caused by the user password, so I tried to disable the password request but this did not work either. This seems to be because of FireVault which I deactivated a couple of hours ago. My system is still encoding so it might take another couple of hours until I can see whether this approach was useful.

Does anyone have another advice?

Original problem: No sound, no micro

My MacBook Pro (2.4 GHz Intel Core 2 Duo, Mac OS X Lion 10.7.5 (11G56)) was on stand-by all the night. When I opened my Mac this morning I had no sound and my micro did not work either (A call via skype failed (I could not hear anybody and they did not hear me) but the camera worked. I cannot hear music via iTunes nor any other application. Also I cannot hear the click sound when one tries to turn up volume.). Therefore I thought resetting PRAM would be the solution but I failed in doing so as described.
Based on what you said, it looks like your PRAM did reset. I usually let it go 3 times before letting my Mac restart. The problem then lays outside of the realm of PRAM. 

Have you tried checking the inbuilt application "Audio MIDI Setup" to confirm what the Inputs and Outputs are? And are the Inputs and Outputs turned up?

If that doesn't work, have you tried doing a SMC reset? Those usually deal with the problems relating to system software.