Im on Safari (and firefox) and have no issues with the sit-a-longs.
double check your version: here
If thats all good then check in your system settings:sharing to see if you have your firewall enabled or if you are using a router, check the firewall on it. Its stands out to me that your running into this on two different browsers, but you can see the operator11 videos. And you say you've got the newest version of flash? That sounds to me like a firewall issue. Also what ver. of OS, Browsers would be helpful.
You have version 9,0,115,0 installed
oh and if you do have a firewall enabled on your mac and/or modem/router - you need to open ports 1935 for both TCP/UDP in/out and the IRC chat port of 6667.