When holding down the keys to reset PRAM, you should see the screen flash between grey and black as long as you hold down the keys. When the screen flashes to grey, you should also hear the startup chime. This is handled by the EFI before the OS kicks in at all, so it should bypass any software issues.
If you aren't hearing this chime, it is likely a hardware issue. Try plugging headphones into the headphone jack and resetting the PRAM just in case you hear some sound coming out. I doubt you will. (The startup chime is always routed through the internal speakers on recent machines whether or not you have a headphone plugged in, which is rather annoying.)
If this doesn't give you any sound, I'm confident you have a hardware problem. Contact your local Genius Bar, Apple support, or local Apple Authorized Service Provider.
