As with video files, there are many ways to create audio files, if you simply want to include a voice over segment to your video. If you are using a Chromebook, (or any other device):

If you are using a laptop or cell phone, most devices have an app or a feature where you can record an MP3. Since there are so many laptop and phone brands out there, the best advice:

Google It!: (Use "your phone/laptop name" + "record audio")