I can't really tell if the sound does not work sometimes or I just missed it. I have lost a few games accidentally on time, although it was some time ago since it last happened. I also would need a more alarming sound. As an example, here is one voice that counts down that I really like:
https://www.youtube.com/watch?v=WlOzFS_t14Y&feature=youtu.be&t=39m15s
There is a special sound (like maracas shaking) if a player is going to time out. I noticed several times that this sound sometimes does not work. I can not find any pattern yet. But it seems that when your time is going down to 9 seconds the sound works. But later if it goes from 9 to 7 (or less) it does not work, or works sometimes.
Did you notice the same?
P.S. Browser: Chrome.
P.P.S. I'd prefer more noticeable (alarming) sound instead of that shaking.