Record FaceTime audio call on Mac
There is no option available natively in macOS to record a FaceTime audio call.

However this can be accomplished easily with the help of a 3rd party software.

You can use Audio Hijack for macOS to record an audio call in progress. The instructions for same are outlined by the developer in this blog post.

As outlined in the blog post, you can record both a FaceTime audio as well as a regular phone call. However, make sure about the legality of the act. You may be required to inform the other party about the recording.

No affiliation whatsoever with the developer, they are very well known for the quality software they make.