What I would try, though I highly doubt this will work since you say you don't hear a boot chime even after resetting the PRAM, is booting with a known good OS, such as the OS install DVD that came with the unit. If you still don't get any audio, then there's always a small chance it's simply bad speakers, but considering there are two speakers, and they have independent connections to the MLB, it's pretty unlikely that both of them would go bad at the same time like this. However, the boot chime does only play on a single speaker, so that is why I suggest trying with a known good OS before getting too far ahead of yourself. It also might be a good idea to go to the sound section in system preferences, and move the balance slider from left to right to see if maybe just one speaker is bad.

Since you don't get any audio from headphones, I don't expect any of that to work, but it'll only take you a few minutes and doesn't cost anything, so may as well be thorough. I would say it sounds like you've got a bum MLB, and given the age of the unit, it's probably not worth the $300 it'd cost to get it fixed under Apple's flat rate program. So my advice would be to just accept this new limitation and start setting aside money for a new laptop. Assuming everything else about it still works fine, no point throwing out a perfectly good laptop just because it can't play sound. However, if that is a deal breaker for you, then I would suggest selling it on ebay or craiglist. You should be able to get $200-$300 for it, give or take, and that would be a nice little down payment on a new laptop.

The sound card, as it were, is really just a couple of chips soldered directly to the MLB, and given the age of the laptop any number of things could have gone wrong, though my money would be on a cold solder joint or several. As electronics age, they dry out, and the plastics become brittle. Same goes for the solder/flux used to bridge the gap between the individual wires on a component and the circuit pathways. Eventually that also becomes brittle and you might get microscopic cracks in it, preventing the electrical signal from making it from component to circuit. That's called a cold solder joint, as opposed to hot, meaning there's electricity running through it. If you happened to work for say TSA, and had yourself access to an X-ray scanner, and were quite capable with a soldering iron, you could probably check for any bad solder joints and fix it up... But I'm guessing you don't have access to that kind of material, or have that kind of skill, because you would have already known and tried this if you did.

Eventually the problems are likely to get worse, which is why I say start setting aside money now. Things could break completely tomorrow, it might be 1-2 years from now, there's no real way to tell. It's probably closer to the latter however, so by the time it becomes pretty much unusable, you should have a decent amount of money set aside for a new one.