Output Internal Speaker with distortion before update to Mojave OS (MacBook Pro 15.6")

Try resetting NVRAM

Reset NVRAM or PRAM on your Mac - Apple Support

If that doesn't work, try resetting the SMC.

How to reset the System Management Controller (SMC) on your Mac - Apple Support

Does it work with headphones? If the sound is still distorted, then it's not the speakers themselves.

I'm also using a 15" MBP, with 10.14.4

I just went and checked to see how my internal speakers are working. I got no sound from YouTube or VLC, and it wouldn't let me adjust the volume. I got an image with a slashed circle. Headphones work fine though. Weird.

Then I went to Audio MIDI Setup, selected the gear icon on the bottom, then selected "Use This Device For Sound Output". I don't remember having to do that before. Now it works.