It's a tricky balance using Microphone and Speakers together. It is easier to combine mic with headphones. If the sound from the speakers bleeds back into the mic, normally this would cause feedback, but modern voice programs have noise-canceling and feedback protection (this applies to WebRTC the built-in voice, also happens to apply on Skype). They will auto-level your mic down, in order to prevent feedback loop. Try to get your microphone away from the speakers, and speakers not pointing towards mic, also turn your speaker volume down. See if that helps. If you've been turning your mic volume up-up-up trying to get louder... perhaps try turning your Speakers Down, and turn your Mic sensitivity Down also. Reduce the amount of sound from speakers that is being picked-up on the mic. Alternately, work on troubleshooting why your headphone volume is too loud. Plug in your headphones again, and use your Computer volume settings to turn-down the headphone volume (since you said the volume knob isn't working to turn down headphones).
