I can't seem to record my MacBook Pro's screen with its internal sound

The OP needs to reroute the Mac's internal audio to his/her recording app (presumably QT). Loopback is the app to use to reroute internal audio to another app (like QT).

Audio Hijack is an audio recorder; it can output audio to a hardware device or a file but to my knowledge not to another app. Fission is an audio editor; it outputs edited audio to a file.

BlackHole Audio is another app that can reroute internal audio.

The native macOS screen recording cannot pick system audio as an input. However, if your Mac has an Intel chip, you could install something like Soundflower to add a system audio option as an audio source. But not that does NOT work with M1/M2 Macs (yet).

As others mentioned, third-party software like Loopback by Rogue Amoeba would let you do almost anything with audio on your Mac, redirecting audio where you want it.

There is also third-party software that does the whole screen recording process, including system audio. I've used Screenflick. It includes software called "Screenflick Loopback" as an audio source, which puts both system audio and in-use microphone(s) as an audio source for the recording.

Here's an article that gives more info about these options, although it's old enough that it doesn't mention Soundflower not working with newer Macs.

https://techpp.com/2020/01/28/macos-record-internal-audio-screen-recording/

aeas94 wrote:

I can't seem to record mac's screen with its internal sound. If I turn on the microphone prior to recording, the surrounding sound is also included in the recording and I don't want that to happen. I aim to record the screen with just the sound of the video playing on my mac. Is there anyway to do this?

Use third party —

Audio Hijack & Fission—https://rogueamoeba.com/audiohijack/