If you have access to the target computer you would just hack it directly to send you the information like an evil maid attack. No reason to do this keystroke sound hooey..

Mine has clicky clicks, because I like it!

Perhaps a speaker in the keyboard that emits a random click-sound to mask the real sound could be helpful here. But it might confuse the typist when it emits the spacebar sound after you typed a regular letter...

I think a speaker is going to sound a bit different than actual keys.

All you need to do is obscure the original sound of the key press so that it can't be distinguished from presses of other keys. You don't need to create an authentic key press sound from scratch

Some keyboards like the “Class 80” have a solenoid or a beeper built in that you can turn on to your liking