The simplest option to write audio from unreal to some file on disk would be using the metasound wav writer node, but if you’d like to capture a full submix this might not be what you’re after, essentially with a little c++ you could write a custom submix effect class that writes the audio buffers it receives to a file or passes them to some external library, which sounds like what you might be after, without c++ though I think that the wav writer might be the only option.