Generally, if you can hear YouTube audio, you are ready to attend.
Since the service we are currently using is browser based, it will grab your default system settings using the JAVA and Flash plugins. So if you plug in a USB headset, you will probably have to change your system defaults and restart the microphone and speakers Flash configurations in the browser window. Webcams often have microphones as do some displays, not to mention headsets.
Check your system, browser, and connection to see if you are webinar ready for either the presenter or attendee roll using the following link. It will run a check and allow you to change any settings before the webinar starts.
http://www.instantpresenter.com/system_test/part_2.asp?bhcp=1