If you can see the other participant's audio meters moving but you are unable to hear them, this indicates that there is a problem with the headphones or where they are connected.
The most common cause of this is that your headphones are plugged into the wrong audio output. Zencastr's output will be sent to whatever is selected as the default audio output device in host computer's operating system.
*There is a weird bug in chrome where the audio output will be sent to your usb mic or audio interface regardless of what is selected as the default.
If you are using a usb microphone or audio interface, check that you are plugging your headphones into the output found on these devices. Sometimes there are discreetly placed headphone jacks on the bottom or side of usb microphones that are easy to miss. Another solution to this bug is to try using Firefox as it doesn't seem to have the same problem right now.