(At the time of writing,) I've tried researching for 3rd-party apps that can record internal audio only, but surprisingly, I didn't find any. The only app that I found which can do this is SCR Screen Recorder, which just like its name, is actually a screen(+audio) recorder. So, while this app might be an overkill, it's worth to try.
Note: you need root access for internal audio recording. Also, the internal audio recording was said to be an experimental feature, which might not work on all devices.
- Download and install SCR Pro (not SCR 5+ Pro, which doesn't need root, but only works on Lollipop and doesn't support internal audio recording)
- Open the app, it will display an overlay buttons on the screen.
- Open the SCR Settings (middle icon, the one with cog icon)
- On "Audio" header, tap Source and select Internal
Optional: if you don't need to record the screen, you can select the lowest setting on "Video" header to decrease the filesize. (e.g. 426x240 resolution)
- Open the app which you want to record the audio
- Adjust the volume from the Android device (important!).
Note: since SCR use the same audio stream as source, the volume when the app plays the audio is used as the input. This means that if you don't record the audio with 100% volume (e.g. 70%), the loudness of the audio will decrease when you play it (e.g. if playing the "recorded audio on 70% volume" with 70% volume, you will hear the audio in more-or-less 50% volume compared to the original audio)
Ideally, if you are using external speaker, you can set the volume to 100% in Android, and adjust the volume to comfortable zone on external speaker.
Optional: turn off any notification on all apps to prevent noise.
- When you're ready, tap the Record button (first icon with orange color)
- To stop the recording, you can either:
- open the app once again (either from notification, or app launcher), or
- turn off the screen for 2 seconds
Addendum by anonymous user:
With root: When the settings have been configured to internal audio and notification icon is popped up, tap the home button (don't close SCR) and open any sound recorder. You'll see that it records the sound internally instead of with mic. (I've tried with 3rd-party HI-Q MP3 recorder, unsure about others..)