In general 'Sound Boards' were added to EM pins, at the operator level. As you state to help the EM compete with the early SS pins. Gottlieb had a kit that could be ordered from them. The other one that has been seen was offered by WICO, there could be other made as well. The board might have a name on it. These usually have more than just 2 sounds.

WICO SOUNDBOARD (resized).jpg

From another Pinsider. This is a WICO unit in a Bally Pin.

I have one on my Target Alpha. It's wired to the chime unit wires.

Found this photo from Google. A 'SAMPLE' Gottlieb Close Encounters with a sound board.
close_encounters_EM9.jpgGottlieb (resized).jpg

I encountered this Space Mission several years ago and always wondered if the sound card was factory installed.
Would the operator have gone to the trouble of adding the sign under the playfield glass?
Space Mission Electronic Sound sm (resized).jpg

Funny that this subject came up- I have a Williams Touchdown game that is missing a knocker for specials and I was thinking of wiring it for a circuit board and speaker that would shout "SCORE!" when conditions were met.

That sticker probably came with the kit. I was thinking about these the other day, never seen one in action. But it might be neat to build something similar with modern tech, using PCM sounds. I suppose if you wanted to get nuts you could wire it in all kinds of weird ways to play music or different sounds when different conditions are met. Maybe somebody has already done this and I can just use their work

Oh god, the memories. peeyew,peeyew,pewew.