soundcard output distorted after bios update how can i resolve this issue?

That's a 64 marker! I reckon if nothing else has changed, it could be the bios.
What you need to know is that the chipset is configurable by manufacturers in ways we normally never hear about.
/begin history lesson
Via had a well known 'hardware problem' with large disk copies in their apollo & mpv3 chipsets
http://www.deinmeister.de/mvp3_zlt_e.htm
http://lkml.org/lkml/2003/8/9/99
This affected large ide copies and sound blaster cards.It was because the settings were tweaked to help in the recognition of creative sound blaster cards.
/end history lesson.

Post your chipset.
If it's Via, they are used to this, and you will get fixes. viaarena.com and other sites.
For the others, I don't know.

You can try...

You can either flash the BIOS back to the version that worked, if there is no pressing reason why the later version is needed.

An alternative is to set the BIOS to its Standard/Fail-Safe Defaults and flash it again. You might then try to change to the Optimized Defaults or adjust the settings individually.

If none of this has an effect, you might try to clear the BIOS. You will have to consult the motherboard manual to see how this can be done.

On some motherboards, there is a Jumper that has to be set, but laptop machines may be different. The manual or Google should help you, as well as a look at the board if you can get at it. Laptops again can be a problem.

thanks a lot for the advise,but i sure'd like to know how i can revert to my older bios,it is an Intel D945GCCR desktop board.

Do you have it...

You should be able to flash the BIOS with an older version. If you have any reasonable luck, there is a copy out there somewhere waiting for you to google.

We are all assuming that the BIOS is the critical variant. Have you tried a few linux live-cds with built-in media support, PCLinuxOS, Mint, to hear if there is any difference?

yes in fact i have installed the latest mandriva distribution, but the sound still has a problem.the sound output on linux is mono when i unplug the jack partly.On windows,sound is normal when you plug the jack into the microphone port(you'll have to configure it), but mono when pluged into the line-out port.

Much as I don't like them, it's rare to hear of intel bios causing issues.
I would try setting options in BIOS and sound config to see if that can sort it,