Each video that has sound will by deafault have an audio track below on the timeline. If the video is in Video 1, it's audio will generally be in Audio 1. On the name tag of Audio 1 there is symbol in it's top left corner that looks like the windows volume control icon. Clicking this will toggle it between 0 and 100% volume. As for exporting audio, make sure the audio is not muted and select file>export>audio. It is only possible to export as .wav, but there are plenty of convertors available to convert .wav to .mp3.