As of now, you can't. You can send recorded voice clips. Although, if you are hell bent on sending audio through WhatsApp, you can upload them to Onedrive and share it with a link - easy peasy..
Thanks for replying. I just found a way to send audio files "mp3". Just go to the files app and change the audio name **type** to a video type. For example, change the mp3 to mp4. And then move the file to the camera roll folder. WhatsApp will send the file from there. Lol