Hi Iceland, welcome to the forums (well, from me anyway).
You didn't list how many people you have tested this webcam with as well as what program you are using. But to hazard a guess,
1) Check the volume setting in the program(s) you're using...
2) They don't have a microphone built into their webcam... not all people have microphones connected to their computers, depending on what they're using...
3) Perhaps they have a setting they need to fix on their end...it could be anything from the sound control in windows to the chat program you're using to chat with them (on their end)
4) If they're using a headset, perhaps they have the mute turned on (some headsets have a button to mute the microphone separate from the software)
It is most likely not something on your side (as you said, you can hear everything), but without more information, it's hard to narrow this down.