We recommend making sure your sound is on. On the left-hand side of your iPhone, there's a small switch just above the two volume buttons allowing you to turn your sound on/off.
Also, make sure your mute button is NOT activated, your volume is turned all the way up and you have a strong wifi connection. :)
If the above doesn't work or if you suspect your mute switch is broken/not working properly, go into Settings-> General-> Accessibility-> AssistiveTouch, turn that on, and when the white circle appears, press on Device and you can unmute.