If you're hosting a room it is best to turn the mic option off as it can kill your bandwidth if you have a slow connection.
If you're not hosting a room you can click on the room members icon at the bottom of your screen (the one that looks like a little man) then you can select the individuals mic options on / off by clicking on their name.
It was possible in GT5, you just scrolled up the participants list, pressed triangle for the menu & clicked on the Mute option. It can still be done on GT6, but PD in their infinite stupidity have made it more clunky & hidden the option away. You now have to click on the members icon in the lobby, where you're taken to a different screen with the lobby members listed.
Strange, as you used to have to do it that backward way in GT5, then they updated it so you could do it straight from the lobby members list. GT6 is sadly a case of one step forwards & 2 back in too many areas, this being another tiny example.
