There is a two ways you can go with for this. First one is to simply to click on the icon (make sure your browser cache is cleared up). You will also need to make sure you are logged in with the correct user that has authority to spy agents.

If for some reason you can’t hear your agents using the browser that can quickly be changed with using a softphone as an extension. Make sure to unclick “use browser to communicate” before login to the dialer.
When you registere a softphone (xlite, deksforcephone, Zoiper) you will go with the same exact operation. Click on the icon and you will receive an incoming call on the softphone. Accept it and you will start hearing the agent talking

With the other two options you can either barge or wisper.

With the barge option you can interfere within the conversation in place.
With the wisper you can talk to your agent while he is on call .