Sound in avi flv mp3 mpg BUT NO Sound in mp4 or Youtube

Everything used to work until three days ago. Now MP4 files and Internet sound has stopped having any sound.
I have updated everything (Firefox, Flash, Java) my Media Player Classic settings
are still OK. I don't know what else to try.

 

please see my reply to your duplicate question.

you can click on your name to get to your profile and see your posts, so you do not need to make a duplicte thread, thanks.

 

First check if your hardware is working - load a live cd like knoppix and try your
audio.
Knoppix is a live cd, copy it from internet the iso and put it on a cd. Once done
you'll have a loading CD - if you start it from your CD drive - find the boot menu
and run it from the CD. Once completely loaded try any sound option - and you'll
know if your hardware is working ! So it's your windows sound that not working or more likely your sound driver that failed or got disconected.