Reddit reviews Mobizen Noise Cancelling Headphones in Ear Earphones - World's First Voice and Internal Sound Recording Feature with Microphone for Kids and Adults Alike, for Android Cell Phones and Tablets Only

I had to use Mobizen to capture the audio and video. To record the audio, I had to get these headphones

No smartphone device can do that on its own. All of them use the external micro to record sound. Actually, you need a special device for that. There is one new kind of earphone that can do that, and are the GameDuck earphones https://www.amazon.com/GameDuck-Mobizen-Recordable-Headphones-Comfort-Fit/dp/B01KZM6KM4/ref=sr_1_1?ie=UTF8&qid=1475219161&sr=8-1&keywords=gameduck+earphone

Hope it helps you ;)

UPDATE: I just tried the Mobizen Headphones with the latest Mobizen Screen Recording app. I must admit, it exceeded all my expectations. It not only allows me to record clear internal audio, it allows me to monitor/listen to the audio while recording internal audio. I wasn't able to do that before even when the app supported native internal audio recording. I'm also able to optionally record my voice pretty clearly. The audio quality of the headphones (for listening) is surprisingly great; although, I still prefer using my wireless Bluetooth headphones for regular listening without recording. Overall, Im pretty happy that I have an app that can do it ALL with no compromises.. other than having to use headphones for audio recording.

I know what you mean. That's why I bought the Asus Zenfone 5z. Runs all games perfectly, and it was a lot less than the other phones that could do the same. And it can also record without lag as well so pretty good imo. An option you might try (which would cost money but not too much) is buying Mobizen earphones on Amazon. I had them before, and they do work well to record internal audio. They cost around $25.
You have to use either the Mobizen app or Du screen recorder to record using them.
(This is all assuming you have an Android device)
https://www.amazon.com/Mobizen-Recording-Earphones-Headphones-Microphone/dp/B01KZM6KM4/ref=mp_s_a_1_1?ie=UTF8&qid=1549957316&sr=8-1&pi=AC_SX236_SY340_QL65&keywords=mobizen+earphones&dpPl=1&dpID=41prs9RB%2BjL&ref=plSrch

You can add this option to your table as well - using a loopback cable. Here are details from Google if you want to make your own:

• https://source.android.com/devices/audio/loopback.html
  
  EDIT: ok, this loopback capability seems to be included in the Mobizen headset that you mentioned:
  
  >You could also buy this headset and use it with a Mobizen app. A "world first" that shouldn't even have to exist!
  > - https://www.amazon.com/GameDuck-Mobizen-Recordable-Headphones-Comfort-Fit/dp/B01KZM6KM4
  
  Basically you plug this into your android device's earphone jack. The output pins are routed to the microphone input pins. So whatever is playing is going to be audible if you record. The quality will be better than off the air. However, it still is not "digital" internal audio as you require - but it will be good enough for everyday use.
  
  One of the ways many Android devices prevent audio recording is by doing some type of heavy echo cancellation even for over the air recording (i.e. using microphone to record what is playing out of the speaker). One of the primary targets of this strategy is to prevent call recording it is generally suspected - so for most North American models, you will find call recording does not work well (there is very heavy echo cancellation type effort to remove the speaker audio from the microphone).
  
  Newer Samsung devices even from non-North American regions are starting to have this limitation (example: Samsung S7 after android update, and S8 etc.).
  
  As other poster reported, android in general does not allow apps to have access to the internal audio - although as you said, on some phones that does seem to work reasonably well (whether that is actually recording digital internal audio, or just the microphone recording off the air and through the body what is being played on the speaker).