I find the countdown bell obnoxious and overly loud. There should be some way to disable the bell that ticks down before a CoC round begins.
In most browsers you can do this on your own.
e.g. in Chrome: Security and Privacy > Site settings > www.codingame.com > Permissions > Sound = Mute
Else, you can mute a tab with Ctrl-m (Firefox and Chrom(e|ium)).