Just a guess, but I think you're listening to your local audio through the screenshare option, making it double for you. However, there is no audio through OBS Virtual Camera. Use a Virtual Audio Cable plugin like VB-Audio Cable, which can redirect OBS's Audio output to act as an audio input for other apps, in your case, Discord. Then, on Discord join a voice channel and click the Video button at the bottom of the screen. This should share your "Webcam", which is actually the OBS Virtual Camera, and make sure your Virtual Audio Cable's output is selected as your mic in Discord's Settings.