1) your friend could have ripped the DVD without any audio
2) he selected DTS in stead of Dolby 5.1
most audio cards/older DVD players can't decode DTS sound.
When I select DTS in a DVD menu, I don't hear any sound either.
If you know someone who can play DTS sound on his PC(new sound cards) you can see if the sound works there.
To be sure, your friend should, when ripping the DVD, select all the sound languages available (normally you are able to select English several times)
Hope This helps.