At the bottom right of the video screen there are three vertical dots. Clicking this will bring up a menu with various options. Click "Settings" to bring up audio and video options.
If you have multiple audio devices, this is where you can select the appropriate one. For example, if you're using a laptop - it may have a built in microphone and speaker. If you're using an audio headset connected via USB, you'll need to ensure that it is selected under the audio settings for microphone and speaker (usually has the brand name in the description - e.g. Logitech).
If you're completely using the built in hardware of a laptop, the setting may say "System Default Device" meaning it's what the operating system is set to using.
There are options to help you test as well. Speaking will make the little dots next to the mic icon flicker. Clicking "Test" next to the speaker icon will make a phone ringing sound. Click Done when the settings are correct.