A few people have commented that their speakers have stopped working after updating to OS X 10.6.4. Some people have had this happen for USB-audio devices like the Apple Pro speakers, and others have had the internal speakers stop working.
If this happens, there are a few things you can try:
Select the speaker again
Many times updates will reset some system settings, including the default audio source. Go to the "Sound" system preferences and ensure the desired sound output source is selected. Keep in mind that this will be the default source for the system, and not necessarily for individual applications.
Check Audio MIDI Setup
Apple has more advanced audio setup options in the Audio/MIDI Setup utility that is in the /Applications/Utilities/ folder. Many times this program is overlooked when troubleshooting audio problems.
Open this program and select the "Audio Devices" tab. Then ensure the default output devices are set, and the properties for that output device are correct. This can include the clock source (internal should be default), speaker configuration (if you are unsure, use stereo speakers), and the format settings as well as the mute check box. A standard format setting to start out with would be 44100Hz, 2ch-16bit (CD quality audio).
The system's PRAM contains some audio settings, so try resetting the PRAM if no other setting changes work. To reset the PRAM, reboot the system and hold the Option-Command-P-R keys all at once. Hold them until the system resets a couple of times, and then release them and allow the system to boot normally. Then ensure you go to the system preferences and set up the audio settings again.
Try Safe Mode
If you have audio plugins, extensions, or other software you are using, there could be an incompatibility with changes Apple made in the latest OS update. Usually the system's audio settings should not be affected, but if there is an incompatible audio-related kernel extension then booting to Safe Mode should bypass it and allow the default audio drivers to be used.
Beyond these suggestions, you can try a rudimentary reset of your audio device settings by unplugging them from your system (especially USB or FireWire devices) and then reset the PRAM followed by running general maintenance routines on the system with the devices unplugged. After the maintenance routines are run, plug in the devices again, update drivers for them (if available) or reinstall current drivers, and set them up again using the Audio system preferences and the Audio MIDI Setup utility.