Hello,
I wanted to create a music visualizer like Monstercat, but every tutorial out on YouTube requires Microphone, but I only want to pick up the audio (mp4) signal.

You would have to gather the visualizer data in a specialized app/program (say, Audacity), turn that into a list, then port to Scratch.

Using MIDI rather than MP4 is another possibility (see MIDI Toolkit)

As far as I know you can't actually check audio files' playing volume, frequencies, etc. directly in Scratch. You could use an actual audio visualization program off Scratch, record the video it outputs, and then upload that to Scratch. That obviously isn't Scratch though. I suppose you could use RokCoder's MIDI suggestion, but from my assumptions that's probably not worth the effort for your purposes; I could be wrong though.

If this is for a small detail in a bigger project, I suggest you just make an animation that LOOKS like an audio visualizer. No-one really cares if those kinds of details are synced.