Hey cakeoaz,
I see that you're having some trouble with the microphone being picked up correctly in GarageBand. I'll be happy to help out.
Make sure that the external microphone is selected in the Sound Input tab. You can check this by clicking the Apple menu > System Preferences > Sound > Input or you can follow these steps: Set up an audio device on your Mac or stream music from your Mac - Apple Support. Also make sure that "Use ambient noise reduction" is toggled on.
If you're still having trouble, test the microphone by creating a recording using the Voice Memos app. Do you hear the same issue that occurred in GarageBand when you listen to a playback of a voice recording you made?
Take care.