Question / Help Audio recording only (FFmpeg)

I'm having an issue with audio recording using the FFmpeg custom output. I chose the container format "adts (audio)" with CoreAudio AAC encoder installed. But when trying to stop the recording, the button is stuck at "Stopping Recording..." until I press it again. Basically, I have to press stop twice before the log will say the recording is stopped. The recorded file exists but cannot be opened, you can see the attached images of me trying to open the .aac file using different software.

Click here for the log.

To clarify what I'm trying to achieve;
I want to be able to stream (as usual) while recording audio only (if AAC support multi-track, then it'll be fantastic).

Why not just record with the same stream settings and making use of the multi-track support?
Well, my goal is to reduce the amount of disk space usage of recording the entire (let's say an 8-hour stream).​
Why do you want audio only? Wouldn't it be useless to not have the gameplay with the audio?
I plan on sampling myself, my reactions when playing a game to use as a soundboard or any stream alerts. It would only make sense that using clean audio would be the best option.

If this can't be done using the FFmpeg then I would just settle with recording using the same stream settings.

Thank you for your help!