Can't record Facebook messenger call
https://www.quora.com/How-do-I-record-WhatsApp-call

This should help you. It's a Quora tutorial on how to record whatsapp calls. Just grab any app that do that, there are a lot of them. The multi audio problem is mainly based on the version of android you're running, not only the phone you're using. For example in android 6.0 there are 3 main audios: notifications, music and allarms. I hope that this'll help you, let me know.