youtube (for instance) - some of the video clips will play but no audio?

hi guy's i asked this before,i have windows xp home,latest media,real,flash macromedia players--all my dvd,cd,etc will play no problems,but when i go onto the likes of youtube some of the video clips will play but no audio??
(all conections are ok)
i've been told i need a codec, some of the ones i looked at are not verified!!
please talk in lay mans terms as i'm not to up with pc jargon.

thanks CB