Every video tile is built client-side via document.createElement, never
parsed from HTML - browsers only seed the live .muted property from the
muted attribute for parser-inserted elements, so the attribute alone
never actually silenced the local preview. Result: your own mic played
back through your own speakers even with no other peers in the call.
Now sets el.set_muted(true) directly whenever the preview's srcObject
is (re)assigned.