Producing to an audio format (like WAV or M4A) like the desktop version can do doesn't seem to be available in the mobile version.
In your case, I'd produce the clip to SD 360p on your SD card to get the smallest file size then use a free converter app like Video to MP3 Converter or Audio Extractor to create the audio-only file. You can then delete the 360p video clip.
The best way to let Cyberlink know that you'd like to see that option added to the mobile app is to use the FAQs & Send Feedback option on the Settings menu or leave a review at the Google Play Store. The devs seem pretty responsive to these kinds of requests.