I am using Logitech Z906 speakers with my computer sound card "sound blaster z" and I use optical connection however after short playing any audio the decode led turn off and sound as well turn off !! to turn it on again I have to open sound blaster control panel software and select either dolby digital or dts . Is there a fix to this problem ?
After checking for any driver or control panel updates the next thing to do is try the speakers on your TV or another PC.
If they work then you have a PC problem.
If they don't work then you have a speaker problem.
From your description I would say it's the soundcard since it can't tell if the speaker has an issue. Having to change the setting to DD or DTS in the control panel is another indicator of the problem being in the PC.