AN engineer made a good point…on his iPhone sometimes there are audio alerts that dim sound when the alert happens. Maybe something similar is happening on your mac? Do you have anything installed that might have these sorts of audio alerts? Even if they are turned down it could cause a dipping issue. Look into the different alerts and notifications and see if this is the culprit. As for wi-fi, I am thinking you would most likely be seeing other issues if it were a wifi issue. Let me know if some kind of audio notification is to blame here.