When I broadcast I always use an external mixer. The main reason is that I find it it's much simpler to handle live then a software mixer running in the broadcast software. I also have a lot of opportunity to add different audio sources.This mixer is my main one for streaming Beringer xenyx X1204usb
This mixer is perhaps a bit overkill for most streamers but I like the faders.
When you mix with an external mixer you use a headset that has it's own volume knob on the mixer. then you use the faders and listen to the result. That way you can mix the sound without actually watching the mixer. You just rest your fingers on the faders and use your headset volume to guide your.
This is perhaps the best way to do mixing. the problem is that what you hear on your headphones might not be the same as goes out on the stream. If your headset volume is set to high in relation to the volume going out if your mixer then your watchers will have a bad day.
You will stand there thinking everything is play while your audience have to crank their stereos all the way up just to get a minimum of sound.
How to get this right?
I would never use a mixer that don't have a output volume level meter. My mixer uses a row of LED bulbs and it works fine for me. This must always be you reference when mixing live.
If you use your output volume meter first and then calibrate your headset volume knob while the output levels are right then you can use the headset as a guide for mixing. But if you change the headset volume you must check you row of LEDs again.
I would also recommend to listen in on the stream from a phone or separate computer so that you can be sure that the levels and content is the same as you are sending from your mixer.
This last part is perhaps even more important if you use a software mixer directly in your streaming software.