@Ruben > There may be some factors that could be causing some noise... Lets run through some of those things:
1) Try using different audio cables in your setup (swap out what you were using for an alternative pair of cables)
2) Try running the setup without your laptop power cord connected
3) Ensure all your firmware and software is up to date
4) The mixer can be used in standalone mode. Try connecting an auxiliary input without connecting your laptop at all and see if this makes any difference.
5) What speakers do you use and how do you have them connected?
6) Ensure all audio cables and power cables are separated as best as possible.
7) Connect absolutely nothing to the speakers... Do you still hear the hiss? What brand / model speaker are they?