The chat client will automatically scan your hardware and plug-ins regarding a microphone, speaker, and camera. Hopefully, all things work out fine, just as intended, and the client will choose the correct sources per default. But in some cases, you will have to calibrate this yourself, if some of the sources are not working.

Go to Settings > General settings.

The easiest one to investigate is the Video Input Source, where you're able to use a preview (see below).
As for the other sources, you may have to try them out one and one, if the input and output sounds are not working correctly.