Troubles with resetting PRAM
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.