If you're sure that your system is working (i.e. you can hear other students), then the issue is probably with the student's device. You should use the chat box to suggest that the student restart his/her device and call you back. Shutting down and restarting solves the majority of audio problems. If the problem persists, recommend the student use another device and call you back.
If you can't hear any of your students, then the problem is likely with your device, and you should try restarting your computer and logging back on. If you are using headphones, you should try unplugging them and see if your computer's internal microphone/speakers can pick up or transmit the audio feed. You can also make sure that Google Chrome is defaulting to the correct media devices on your computer by going to the tutor checklist (https://www.cambly.com/en/tutor/warmup) and clicking on the camera icon on the right side of the URL address bar.
Still having problems? Click "Submit a request" on the Help Guide!