What can I do to enhance MacBook Pro internal mic sound quality in iMovie?

Okay, so eventually I did figure out what was wrong.

Of course, I'm not saying what I got is as good as a decent mic, however I find it much better now, even suitable.

What you'll want to do is go to System Preferences, choose Sound, select Input tab and ensure ambient noise reduction is unchecked for the internal microphone:

Apparently, this option is enabled by default and controls a filter that distorts the sound that goes in through the mic. It may be good for noise reduction but in my experience recording is so much better with it turned off.

After you've killed the main culprit, go to iMovie, click button with a mic on it, and turn off Voice Enhancement:

This may not have as dramatic effect but still I prefer the sound as raw as possible because Macbook's default processing somewhat disappointed me.

Also you should make sure that the sound level is not off-scale when you're recording. Note that iMovie input level setting is bound to System Preferences, i.e. when you change on of them, the other changes as well.

That's all I found so far to make Macbook Pro internal mic sound better.

Of course the Pro isn't going to be much better. Dollars to Donuts says the Mic in it has the same part number on it as the one your old Macbook had. The "Pro" doesn't refer to its audio recording abilities.

My brother-in-law does the same thing with his Macbook. We ended up buying him an Audio Technica USB Condenser Mic. Works like a charm and the audio is head and shoulders above what the internal mic is capable of. If you're serious about recording, $100 is a pittance of an investment, especially when you just spent $3k on a 17" Macbook Pro.

My advice is to buy a real mic, however there are few settings, you could tweak.

They are located in the Audio MIDI Setup application (nothing to do with MIDI) - format (8kHz-92kHz, 16-32bit) and loudness.

The mic itself is located under the left speaker grille, you could try different recording distances, or disabling sound output from the speakers while recording. Closing or opening display a little could serve as sound reflector.

I don't think this could fix a crappy mic, but it's worth a try.