- Please make sure you have set up your audio device and its ASIO driver in Options->Preferences->Audio->Device and selected all inputs and outputs (it's a range selection requiring you to set the first and the last in/output to be used by REAPER). - Setting up the device is described in section 1.12 (page 22) of the User Guide (please get it here if you don't have it yet: ) - Also make sure no other application using audio is running (this includes web browsers etc.) which could steal the device from REAPER. Other than that, I'm afraid you have to be a bit more specific on what you've tried so far and how exactly it doesn't work.