Use HomePod as a microphone for a audio recording? I know that you can send a call to a HomePod to use the microphone on the HomePod to talk to the person you're calling to, and use its speaker to listen to the person calling. Since it is possible to call others with the microphone on the HomePod (through the iPhone of course), is there a way to do audio recording with the HomePod's microphone (with some app like Voice Memo)?
Unfortunately no. There exist no official way of recording audio from the HomePod into for example Voice Memo or other "generic" apps.

You could use a call recording service or voice mail system to record audio from the HomePod by dialing into that system. It would be impractical and the quality wouldn't be great, so it is probably not a realistic option for you.