If the following suggestions work, please let me know and you can Accept the answer (and if possible, leave positive feedback). If not, please DO NOT Accept the answer. Instead, let me know and we'll try some more things or I can refer you to another Expert who may be able to help you further.
If your system is running 10.5.4, that is actually Leopard (not Snow Leopard). The most current system is 10.5.8. I suggest you back up your system to an external hard drive using a program like Carbon Copy Cloner (free). Then update to 10.5.8.
I suggest backing up first, just in case the update causes some problems with your system for whatever reason (download fails, install fails, conflicts with your programs, whatever). I personally do this everytime I make any changes to the computer. It creates a clone of your hard drive onto an external. That way if something goes wrong, you can restore back to it's original state. I do it on all my machines weekly.http://www.bombich.com/
Do you have a set of speakers connected to the Airbook? If so, it is possible that the computer encountered an error at the headphone port. First, unplug the speakers from the headphone jack. Go to System Preferences / Sound / Output. What does it have selected as output? It should say "Internal Speakers". If it doesn't, then put the headphones back in, pull them out. Does it still say "Digital Output" or "Headphones"? Try putting the headphones in and out a few times to reset the port.
Also, check to make sure "Mute" is not checked at the bottom.
If the above doesn't correct the problem, then let's try a PRAM reset.
1) Shut down the computer.
2) While the computer is shut down, locate these keys: command (has an apple on it), option (sometimes called alt), P, and R.
3) Turn the computer on, then press and hold Command-Option-P-R before the gray screen appears.
4) Hold these keys down until you hear the startup sound a total of 3 times.
5) Release the keys.
6) Do you have sound now?
Please let me know how this works out. I'll continue to help if necessary.