Exactly what Aaron said
But to expand Though for non-native english speakers, a good place to start is checking if you are using OBS's Desktop Audio as an Audio source. in the original post, my suspicion is that even though muted, that mute is, as Aaron mentioned, AFTER OBS Studio already got it (hence hearing live-delayed, therefore repeated audio/echo)
For this reason, and to simplify my life, and my use case NOT needing Desktop Audio, I disabled Desktop Audio in OBS' Global Settings. I added only those specific Audio Sources (and never Desktop Audio) into appropriate OBS Studio Scenes. This worked for me, as my use case is one that doesn't need or want anything from Desktop Audio (only pre-recorded content, and externally connected mixer & mics... all of which added as specific sources)