the most obvious possibility is ..... open volume control
you should see a number of categories (which ones vairy)
but
"Volume" - "Wave" - CD Audio" - " line in" will be on the list
....... guess which one you should check ?? ;-)
(don't forget the mute option)
alternatively .... your soundcard may have a custom set of controls)
if not that
what format are the files that will and won't play?
the drivers sound OK .... that leaves codecs
if the "on PC" and ipod files are mp3
and the "silent" files are "propper" comercial "records" .... it could be a codec problem