sound disappear duryng play

I was playing a divx movie and the sound disappeared and I got some squiky noises.
I have an onboard sound and I installed knoppix 5.1 on the hard disk and I did not have any problem with the sound until today.
I try sound card configuration but it is not detecting as say it is not going to work with pnp card that'll be automatically detected.
I try to make some change using the control center sound system, but nothing change.
If I boot from cd the sound is working, and if I restart the system and start to play a sound it is going to work for few seconds and after the noises are back again

It may be that the digital sound buffer is being emptied too quick, I've heard this described as sratching or skipping noises.

From kde control centre, sound system see the section "skip protection". Move the slider to the right this will use memory and buffer
the sound stream.

With no other details it is difficult to answer your question