I have a Dell/Zylux A425 2.1 audio system for a Dell PC. The speakers don't seem to turn on at all - no sound, no pops on power up and no indicator light on control speaker. Circuitry shows power at output of bridge rectifier inside the woofer (about 26VDC). Is there a schematic available or a part that is most likely bad? Quad amp? other regulator circuitry?

It seem your amplifier doesn't work, nothing to do with your computer, you can check with any other source lik eMP3 player, etc. If you take apart the box, there are some SJ4558 dual opamp. In my panel three of them were not working. Check if they are working or not. TDA 8947J is the power amp ic, download the datasheet.
You can also check the input PINs there are article about it.
After I replaced the 3 bad opamp, worked fine like new.

Necesito los controladores para el subwoofer a425 esque compre la bocinas pero no me las lee porfabor un link o si tiene los controladores fabor de enviarmelos al correo [email protected]

If your computer came with a motherboard disc the sound drivers could be on it
click start control panel administrive tools computer management device manager scroll to sound,video and game controllers you might see a yellow question exclamation mark? ! or a red X Right click to reinstall drivers
click start control panel sounds and audio,volume,advanced,speakers make sure you have selected the
(((appropriate speakers))) also audio you will find 3 drop down lists make sure you have the correct settings there
also
click start control panel sounds and audio devices select advanced there might be a tick in the mute box untick it
if his does not fix the problem
click start control panel sounds and audio devices you can adjust the volume there also select speaker volume you can change your settings there you can also tick in the box place icon in taskbar for any further volume control settings you will need to right click on the volume icon system select adjust volume

click start control panel sounds and audio,volume,advanced,speakers select the appropriate speakers from the drop down list or navigate to the device manager scroll to system devices + to expand,system speaker right click properties then select use this device (enable) also you might check
the speakers plug/outlet is usually at the back of the computer you will have to open up the computer to check the lead attached to the rear of the input plug make sure the connection is good
try cleaning your computer of dust using a fine air blower from a compressor making VERY sure that there is no MOISTURE in the air line specifically around the sound card and the CPU central processing unit dust causes static to build up in a computer

if you dont have a motherboard disc you will have to download the drivers
if you need help downloading sound drivers post a reply
hope this helps you