If everyone can hear and see the presenter fine except you, it's probably something exclusive to your network/internet or your device.
We recommend the following:
Is your computer muted?
Are your speakers/headphones/speakers working properly/configured correctly? Check that they are working correctly.
Close any other tabs/programs you may have open that may be using your internet.
Close other programs that may block your computer (Skype, GarageBand, Hangouts, FaceTime, or other similar programs).
Check your internet and the technical requirements for students/attendees.
Restart your computer frequently and make sure your antivirus and browser are updated.
We also suggest:
Leave the virtual room and re-enter
Restart/change browser
Restart your computer
Restart your modem/router
Try connecting with a network cable to provide more stability and improve your Internet.
Try connecting in a place where you do not share a network. Sometimes being in public places like coffee shops, etc., where the Internet is shared with more users can cause the speed you receive to be very slow or not enough.
If you are in an office, they may have very strict security protocols; contact your office IT representative.