After you sat Line 1 as your default playback device, you did not hear anything because the sound goes to this device.
As the second picture in the guide shows, in the Recording options of Windows you open the properties of Line 1, and activate the Listen to option. In its dropdown you select your normal Headset/Speakers device.
Now you should hear music, browser sounds again on your headset. In Mumble make sure you select your Headset/Speakers as well, so it does not send any sound to Line1. And in OBS just make sure either default or VAC Line1 is selected as your desktop audio device.
And your done, Mumble is excluded from your stream. You only have to think about how you want to use your mic on stream. It could be strange if you answer people on Mumble while your viewers can only hear you :)