Make sure that the 1818 drivers are up to date
http://www.presonus.com/support/downloads/AudioBox-1818VSL and also afterwards go into UC Surface control panel and make sure it's firmware is up to date.
f you don't hear any sound look at the unit and see if the meters are showing activity, if they are, check your speakers and connections to your speakers and make sure they're plugged into the correct outputs. If you are using headphones the outputs are 7 & 8.
Assuming that your 1818 is synced to your system correctly, has the latest drivers, has the latest firmware, and the master bus is assigned to the correct output, it should work. If it doesn't make a support ticket so that a support agent can assist you.
