I have a problem when playing Portal 2 online with my 11 year old nephew--namely that I'm hearing a echo of my own voice with about a 1 second or so delay. It drives me nuts and I don't know what would be causing it.
Facts on the ground: my machine is a Windows 10 PC on a Z170-A motherboard with some flavor of Realtek built-in sound hardware. My headset is an older Sennheiser PC150 (I think) with separate analog mini-jacks for mic and headphones, plugged into the front headphone/mic ports of my FD Define R5 case. He's on a Macbook Air of some kind, with a Turtle Beach headset (I assume USB).
We're just using the in-game Portal 2 voicechat, not the Steam one (when we switched to the latter for some reason he wasn't hearing me anymore and we gave up).
Anything I should be checking for? He says that on his end he's got his headset on, so I don't think that's the issue. Is there a chance that somehow, since we usually start by chatting a bit in the Steam client, that Steam on my PC is still listening to my mic, then the game is
doing so separately? He always ends the chat on his end beforehand, so I've assumed that that's enough.

Any help will be apprecited.

I didn't find the right solution from the Internet.

Digital production studio

Thank you.