Download and install BlackHole 2ch (you don't need the 16ch version in most cases).
Open Audio MIDI Setup on your Mac in Applications/Utilities.
Create a Multi-Output Device by clicking "+".
Check the Use boxes ☑︎ to add your soundcard as the Master Device (which will be used to monitor your sounds, here it’s a Lynx Aurora(n)) and BlackHole 2ch to the Multi-Output Device. Also check the Drift Correction box ☑︎ for BlackHole and all Audio Devices that are not the Master Device.
<aside> 💡 For the sample rate, it depends on your DAW settings, if you’re used to work at 44.1khz or 48khz or higher. It’s recommended to set the same sample rate here and in OBS (go to Settings > Audio).
</aside>
Go to your System Preferences / Sound and select the Multi-Output Device as your main Audio Output. Now you can play sounds from any of your Mac application, send them to OBS via BlackHole and monitor those sounds through your soundcard output.
In your DAW, choose the Multi-Output Device as your Audio Output Device.
in Ableton Live
in Logic Pro
Create one or multiple tracks for your external instruments :
In OBS (check 5. How to set up OBS? for more info) :
Go to Settings and select your Internal Mic in the Mic/Auxiliary Audio menu (Mic/Aux will appear in the Audio Mixer window).
Add an Audio Output Capture in the Sources window by clicking “+” and choose BlackHole as the Device. Feel free to rename it.
Adjust the levels between Mic/Aux (your voice) and the Audio Output Capture (your DAW and all other audio sources) **in the Audio Mixer window. Note that you won't hear your mic, the easiest way to check levels in that setup is to record the OBS output and listen back to the file. You can also check sound levels before going live in your session cockpit but there will be latency (read the section is FAQ about adjusting sound level for details).
<aside> 👉🏼 Don’t be afraid to go in the yellow and even red on the OBS meters otherwise your sound may be quite low for viewers.
</aside>
This requires a computer and soundcard powerful enough to have low latency in your DAW.
Example in Ableton Live.
NEXT STEP : 5. How to set up OBS?