How are you trying to do it? The Virtual Camera is silent. Several requests already to add audio to it, but OBS's audio is such a mess already that the devs are looking at a complete cleanup. I wouldn't expect anything to change until that happens.
Anyway, you'll need to use OBS's Monitor as its audio output, and then send that to a loopback driver for Zoom to pick up. Install the loopback, and you get a virtual speaker that you can send OBS's Monitor to, and a virtual mic that you can tell Zoom to pick up. Whatever goes to the virtual speaker, appears in the virtual mic.
This one is popular for Windows:
VB-Audio Virtual Cable and App's
vb-audio.com
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If you're already using OBS's Monitor for your headphones or local speakers, then you'll either have to give that up, or do your audio work *outside* of OBS.
VoiceMeeter Banana, the Advanced Virtual Audio Mixer by V.Burel
vb-audio.com
VoiceMeeter Potato, the Ultimate Virtual Audio Mixer for Windows
vb-audio.com
Those are two different sizes of the same virtual audio mixer. They can take physical sources like your mics, and virtual ones that work like the simple loopback above, except that *these* virtual devices connect to that mixer instead of directly to each other.
- Use the virtual speakers to capture other apps, so they appear in the corresponding inputs to the mixer.
- Use the virtual mics to feed Zoom and other apps, fed from the corresponding outputs from the mixer.
- Use a physical output from the same mixer, not OBS, to feed your headphones/speakers.
If you have videos in OBS, or other soundtracks that you can't move out of there, send them to OBS's Monitor, and send that Monitor to one of VM's virtual speakers, so it appears in the corresponding input to VM.
If you still want to record or stream in OBS, then one of VM's virtual outputs can feed a single audio source in OBS for that purpose, through the virtual mic that that output of VM corresponds to. Do all the work in VoiceMeeter, so that that single output, and source in OBS, has everything in it that you want to stream or record.