midi keyboards have no sound modules, can't play sounds, don't understand what you're saying.They are just controllers.Again, midi keyboards (just a type of midi controller) produce no sounds.
Not really,
years ago, there were synthesizers.
These synth used it's electrical power supply to generate sound. These sounds where raw, but after a while the presets became more and more capable to produce sounds that sounded more like accoustic instruments, depending an the onboard memorie capacity.
People started to buy more synths and wanted to connect the devices together.
The large manufacturers of these toys came together and created a program language called MIDI.
Meanwhile they produced keyboards that were capable of reproducing conventional instruments, like piano, trumpet and snaredrum.
If they confirmed to the MIDI protocol, it was called a MIDI keyboard, not to be confused with a keyboard used for typing on a computer.
Then the PC hype broke loose. And when finally the PC was able to use software affordable enough for the masses to sequence MIDI data along with audio and the internet happend, the name of MIDI keyboard toke a turn (or so it seemed).
Now those same manufacturers make money by selling VST's and software. To more you pay, the more you get quality sounds. That's why they try to make you believe a MIDI keyboard has no sound, while in fact they try to sell you just a controller.
Still the mayor brands sell MIDI keyboards with onboard soundmodules, and they are of great quality too.
But to create a new market for the next generation they try to keep that one as quit as possible.