hello i had the same problem.

my first tip would be : - try asking in a media specialist shop such as panasonic, currys etc, (i dont know the equivalents in spain) explain the situation and see if they can help.

I got audio on my pc monitor by getting some specialist cables: a 2 end lead into 1 end lead sound converter.

a 2 end - 2 end audio bridge/converter which you can then plug the 2 xbox sound jacks off your xbox pc monitor adapter and get sound into your xbox.

to clarify - the first component listed - the single end goes into the pc monitors audio slot, the 2 ends of that same cable go into the 2 end bridge, and the xbox audio leads go into that to connect it all up.

hope this helps!