If you want to set audio levels in QLab while not being patched into the theatre sound system, you are only able to work with the 2 channels of your built-in computer audio. However, if you want to set preliminary levels for more than just the first two channels, you'll want to download Soundflower.
Soundflower is a handy little application that tricks your computer into thinking it is connected to an audio interface. Once you install Soundflower, QLab will recognize it and will allow you to patch to it. It has a 16 channel option and a 2 channel option. Use the 16 channels.
Of course, no sound will come out because it is not actually a piece of hardware. But you will be able to make a guess and set some preliminary levels on multiple channels.
