Generally you would want to use one of the stereo mix buses to send to the live stream. Patch that mix to channels 1(L) & 2(R) of the USB outputs. Most devices aren’t looking for 32 channels, so they only look at the first 2 for audio. As an alternative you can also assign the main L/R to channels 1 & 2 of the USB outputs. Doing it this way, it will follow whatever is coming out of your main speakers.
Using a mix is better since it gives you the ability to control the live stream separately from the main mix. Setting the mix bus to POST FADER on all channels, then starting with all faders at 0db in the stream mix will make a copy of the main mix. If something appears too loud in the stream, you can select that mix and turn down those specific sources slightly. They will still track the main mix, but be slightly lower in level in the stream. Doing this makes it easier on the sound tech since they don’t have to manage 2 entirely separate mixes.
