Right, you have no
need to, but playing all audio on the same device is the
easiest way to solve your stream problem. Another option is to go into your Recording Devices > Stereo Mix for your discrete card > Properties > Listen and check the box and select your Onboard audio to listen to it. Then Mumble will also play out your speakers, but it will also play in your headphones with no other game audio. That's probably not idea either.
If you don't want to change your current audio setup at all, then you'll have to buy VAC and use that, and even then, it will be a sub-optimal setup until OBS adds support to be able to change the source of your Primary audio.
To use VAC with your setup, here is what you need to do:
- Open VAC Control panel and create 1 Virtual Sound Card (Line 1).
- In your recording devices, go into the Properties for the Stereo Mixes for both of your physical sound cards, and your microphone. Go to the Listen tab and check the box to Listen to this device, and select Line 1 for the device that should listen to it. (If your stereo mixes aren't shown, right-click -> show disabled devices and enable them). So all 3 devices (2 Stereo Mixes and 1 Mic) should all be listened by Line 1.
- In OBS, Mute your Desktop sound and change your Microphone selection from your actual microphone to Line 1.
If it all went well, I believe this will get you what you want. It's not ideal, but it should work.