My sound turns off after a while of working with PC, mostly after playng games, sometimes it comes back for seconds and then goes off again. I have an onboard soundevice. And Rodeon 9800 Pro 256 mb Video, maybe there is some connection with sound. I tride to reinstall driver did not help it all. So is it just bad soun device?

Yeee and other thing. The only way to get it back to normal is to reboot PC, and when I reboot it when that Windows XP screen of shuting down pops up the sound is there.

It does sound like a driver issue. The fact that it works ok after reboot pretty much rules out a hardware problem. Does it happen with particular games, or is it any app that will cause the problem?

Many games can cause problems with graphics and sound drivers, so it may be worth checking the game sites to see if there is a known problem.
Another thing is to try different ways of shutting applications down - sometimes the game (or other app) has taken over the soundcard and won't let go - check to see if there are any options for adjusting the sound settings within the game, and try backing out of the games gently (i.e. go through the menus as opposed to simply clicking the 'x' box to close things down)

As far as the sound drivers go, check to see if your motherboard manufacturer has any updated drivers or motherboard utilities.

Ye this problem is known for people in gaming world, but seems that nobudy have a solution. And most of the time it happens to PCs with Radeeon video cards. Somebudy trying to blame onboard sound device, and I want to try to put sound card in my box but I dont know if it gonna help.

Do the sounds for Windows events play as normal:
1. In Windows normal mode?
2. In safe mode?

I allocate sounds for "Open Program" [beep] & "Close Program" [whoosh] events, so I hear the beep, beep , beep, [whoosh] of programs opening [and some closing again] at startup.
Then whoosh, whoosh, whoosh, etc at closedown.
So I always know whether my sound is working.
Even when the screen is black [perhaps something has gone wrong] I may hear a program closing [screen is black and I've used "Ctrl+Alt+Del"] or opening [after restart from standby].

These would let you know that there is nothing wrong with your sound system's ability to make sound.
It would then probably be an inability of an aplication to generate sound.
I was wondering if another application was interfering.
Perhaps you could use "msconfig" to disable all the startup programs [to see if one of them is indeed causing trouble], and then "enable by halves" to find the culprit.

Have you checked "Device Manager" when the sound isn't working to see if all's well there?
I believe Windows can dynamically disable hardware.

If sound cuts off it cuts of complitly with Windows sounds. But what I found is if you do a certan taskthe sound comes back, I mean in one game after sound cuts off if I minimize it sound comes back till cuts off agan, on another game difrent task brings sound back .... It weard isn't it?

Some games by-pass Windows and take direct control of the hardware.
[Rather selfish behaviour to take control of sound away from Windows (and therefore away from all other applications working under Windows)]

If the game was mis-configured sound-wise, or wasn't working properly, then perhaps only that game [and any other that had similar failings] would fail to produce sound. When it's in operation it grabs control of the sound card and you get no sound, then when you minimize it, it relinquishes control of the sound card and you get your sound back.

This is just guesswork by me. I don't know if it's right.

Have you tried working in Safe Mode?
If you have no sound problems when none of the extras are running, then probably one of them is interfering in the normal production of sound.
That's why I suggested using "mscongig" and "enabling by halves".
Have you tried anything like that?