The 2 "dings" you hear correspond to the "flashes" on the MPU and it's how the members above determined the possible culprit

You're getting the 2'nd flash but not the 3'rd
https://www.pinwiki.com/wiki/index.php?title=Bally/Stern#Bally_.2F_Stern_MPU_Board_LED_Flash_Sequence

Can you provide an image of the circuit boards in the backbox ?
looking to see if the original battery is there that should be removed
https://www.pinwiki.com/wiki/index.php?title=Bally/Stern#The_Single_Largest_Culprit_in_MPU_death

the Stern MPU-200 board would use a kind of special adapter board

Pinside member Barakawins1 seems to only have it listed as a single unit on his web site but not here on Pinside or Ebay so I dunno if you would need to resort to another "battery removal technique" listed in Pinwiki

https://nvram.weebly.com/

scroll down to:
Stern MPU-200 NVRAM adapter.
One adapter replaces both 5101 RAMs at U8 & U13

I looked at the site & it says Stern MPU-200 is supported, I don't know if you can use just one card or if you would need two http://lockwhenlit.com/anyPinlist.htm ,