is it possible to be heard when you haven't join your computer audio?

Hey there,

I joined a meeting without joining my computer audio - I just closed the join computer audio window when it came up so the 'join audio' icon with green arrow in headphones was in the bottom left as per screen grab attached.

I assumed people on the call could not hear me. I was then asked to mute my mic by the admin. Were they just noting my mic wasn't muted visually before the talk I was attending began or could they actually hear me? 

Thanks!