Some folks were talking about adding digital sounds to an early SS machine, so I gave it a try with my newest revision of Stars2020. If you're not familiar, this project updated the rules of Stars (1978) by adding an Arduino Nano to J5 of the Stern MPU100. The goals of that project were:
1) Deeper game play (3-levels of stars, wizard mode).
2) Modern features (ball save, tilt warnings, free play, etc.).
3) Cheap (I worked to keep the hardware costs under $20).
4) Leave the original hardware intact (newest revision allows for dual boot - old & new).

With those accomplished (documented here: https://pinside.com/pinball/forum/topic/stars-2020-new-code-for-stern-stars-1978), I expanded the scope a little and spent $44 on a WAV Trigger board. The WAV trigger is plugged into a serial line on the Nano and it drives an old set of speakers I had lying around.
Here's a video I made of the result:

Shoot, I was supposed to drive a circuit board down to you. It must have slipped my mind.

WAV Trigger is here:
https://www.sparkfun.com/products/13660

Code, build instructions, and manual here:
https://github.com/MrEkted/BallySternOS

Wav zip is here:
https://drive.google.com/file/d/1rbLA-aZqXULq8KrAlTwHowQ2WFO9VbFQ/view?usp=sharing

Edit: the zip of wav files has moved here - https://drive.google.com/file/d/10W-ejHtfAnJoimFzfkmMeYZWIZv4Plgj/view?usp=sharing

Wow - cool. I just checked in code that I've been tweaking today. If you have the capability, you might want to re-flash your WAV Trigger up to 1.30b if it's not already that or greater:
https://robertsonics.com/wav-trigger-downloads/

I don't have a way to flash mine right now, or I would. Version 1.3 adds the ability to "lock" a looping voice (like the background music). I noticed that if you rip the spinner, it plays enough sounds to stop the background music. So, in my latest check-in spinner sounds (and bumpers, 10-pts, rollovers), will stop previous instances of themselves before starting a new one. It works, but it sounds better stacked so I'll put in a compiler switch when I get the latest WAV Trigger firmware loaded here.

Let me know if you have 1.3 or greater, and I'll put up a version for you to test that takes advantage of that capability.

If you get a chance, post a video when you get the sounds up and running.

Yeah - you have to track down the wavTrigger.h where it's installed. There are instructions somewhere on that site, but I'm not seeing them at the moment. I believe that by default, you should find the library installed in:
Documents > Arduino > Libraries > WAV-Trigger-Arduino-Serial-Library-master

Then if you update that wavTrigger.h, it shouldn't need the AltSoftSerial.

Two reasons:
1) I don't want a dependency on the WAV Trigger library for people who don't intend to use it. It can be a pain to install & configure that library and if you don't have the hardware, there's no reason to go through that dependency.

2) Size is an issue. Compiled for both Chimes & WAV Trigger, the code is now 97% capacity. That really limits future changes to the underlying OS & bug-fixes, etc. I've recently pushed an update where WAV Trigger and Chimes can be compiled as either or both, so if someone wants the option it's there. On a machine with both enabled, the Music Level setting goes from 0-5.

I'm down with that idea (voice) -- a friend said the same thing the second he heard it.
I just need to find someone who sounds good & has a decent microphone and everything.
I know a couple of v/o people. I'll ask around.

I've been thinking the same thing.

Glad you got it working. If you end up tweaking any of the other sounds, let me know.

Excellent, I managed to get mine flashed also. I put it up on GitHub. Grab the latest and uncomment this line in Stars2020.ino:
#define USE_WAV_TRIGGER_1p3

It's a subtle change. With the new firmware, I can tell the WAV Trigger to keep the background music playing while using the other channels for the sound effects. That way I don't have to worry about a bunch of spinner sounds clobbering the background music. You'll hear the sound effects mix together more clearly with this update.

That's awesome. As dothedoo asked, what's behind the glass? Looks like a spiraling RGB LED light show?