I don't know much about Sterns but there must be a sound board on Monopoly, I recently bought a Jukebox and the guy did a complete restore on it before selling it to me, he changed the caps as well, he told me that the sound will be as if I had a new Jukebox and sound is great (it's a 1977 Jukebox) if it were me, I'd check if you have caps on that sound board and if so, I'd change every single one of them.
Just yesterday I was doing some research on capacitor life span (because I'm having a small problem with my Pin2K monitor these days) and electrolytic capacitors (the ones that are in our games.....) have a life span of 1000 hours to 10 000 hours, if the game was turned on 24 hours a day at a certain time of it's life and that Stern put in cheap caps (which I think is the case), then, 3 years of this treatment and most of the caps are'nt good anymore.