Why does the pitch of my MacBook Pro changes when i plug something off the headphone jack? I've been wondering about that for quite a long time already.
My MacBook Pro (Retina, 15-inch, Mid 2015) runs on Mojave (10.14.4) and whenever i'm plugging off my headphones from the MacBook to switch to the internal Speakers the pitch of the outcoming audio is about 3 semitones up and after about 15 seconds it switches back to the original audio signal. 

Honestly i don't remember for how long this problem is occurs but i think i noticed it first after a major software version update.
As i'm developing myself i'm just curious how something like this can happen, it makes no sense to me at all. (I can only guess it's a software driver bug)

A good friend of mine was also running a Mid 2015 Macbook Pro 15" and he never had that problem.
Try opening the Audio MIDI Setup app

and adjusting the Format of the External Headphones or the MacBook Pro Speakers so they're the same sample rate (48,000 Hz in this screenshot).

Mine were not the same.