Click settings, just below the algebraic notations, there is an option there to highlight the last move made. Also, you can set up email alert if it is your time to move during daily chess. I am using my smartphone and it makes a sound if my opponent makes a move. So i dont know about the sound issue in imac.
sound issues

Eb_orange, I too am having the same sound issues. This seems to happen for me at fast time controls. Sometimes the sound just quits completely. I was restarting my whole system to fix it but found a fast way to fix it. I am not sure how this works on an iMAC but for any other members having the same problem this is an easy fix. For me, I am using firefox/windows vista. So I hit the start button and type services.msc then when it opens I scroll to windows audio, right click and then restart. This restarts audio without having to reboot the entire computer. Then I just minimize that window as the sound will fail again at some point and re-do the process. This has been happening or quite a while and is very annoying, Chess.com is the only site where I lose sound.

This happens to me, too. There is occasionally no sound when my opponent makes his move. This is a liablity in speed chess and sometimes cost me a minute or two a game because I'm concentrating on the position while he's thinking. Only he's not! He's already moved!
I found this can be easily solved by clearing cache in a following way:
1. Close all chess.com sessions
2. Go to Chrome - Settings - More Tools - Clear Browsing Data - check "cached images and files" and uncheck everything else. It looks like sound files are improperly cached sometimes.
You can do it in a similar manner for Friefox and IE, just search how to clear cached data.
I have an issue with no sound when my opponent makes a move although there is sound with my moves. I am using an iMac, any help would be appreciated including another method of alerting me to the opponents move. As the board is basically one dimensional, moves are not readily seen. Again, any help would be nice.