Are you watching your own stream?
When system audio is added as an audio source, any sound played through your system will be heard on stream.
For example: If you need to watch your stream, be sure to mute the audio or you will experience an acho.
Microphone too close to the speakers.
Microphones can pick up audio coming out of your speakers or headset, causing an echo.
“Listen to this device” option is enabled. (Computers running Windows)
Open Sound options by clicking the Start button and type “Control Panel” and click on it. In the Control Panel search box, type “sound” and then click on the result “Sound”.
Click the “Recording” tab, then identify your microphone. Once you identify which option is your Microphone, click on it and then select “Properties”.
When the Properties window opens, click on the “Listen” tab and disable the option “Listen to this device” if it is turned on.
Two microphones set up.
Most webcams have a built in microphone. Upon initially setting up your stream settings, it is not uncommon for the wrong microphone or an extra microphone to be added in error. Have a look at your audio mixer and ensure that the microphone you want active is the correct one (and that there’s only one of them).