Pin Vol external volume control for Stern machines. I bought this mod to use on an AC/DC machine but ever installed it. $25 shipped in the US.

This mod enables one to adjust the volume on most Stern SAM, SPIKE, & SPIKE 2 system pinball machines without opening the coin door. The unobtrusive switch is mounted in the extra opening just below the start button (if a tournament button is installed in this opening it will have to be removed). This button can be used anytime when the game is on or being played.
Installation is simple and no permanent modifications to your game are required. Complete instructions are included.
Please note:

This mod is only for Stern machines that have an existing square recess or plugged hole just below the start button on the front of the machine.

Compatible Game List:
Wheel of FortuneShrek
Indiana Jones (Stern)
Batman The Dark Knight
CSI
NBA
24
The Lord of the Rings
Iron Man
Big Buck Hunter
Avatar
TRON
The Rolling Stones
Transformers Pro/Premium/LE
AC/DC Pro/Premium/LE
Metallica Pro/Premium/LE
Avengers Pro/Premium/LE
X-Men Pro/Premium/LE
Star Trek Pro/Premium/LE
Mustang Pro/Premium/LE
The Walking Dead Pro/Premium/LE
Game of Thrones Pro/Premium/LE
WWE Pro/Premium/LE
Kiss Pro/Premium/LE
Spider-Man Vault Edition
Ghostbusters Pro/Premium/LE
Batman 66 Premium/LE/SLE
Aerosmith Pro/Premium/LE
Star Wars Pro/Premium/LE
Guardians Of The Galaxy Pro/Premium/LE
Iron Maiden Pro/Premium/LE
Deadpool Pro/Premium/LE
The Munsters LE/Premium/Pro
Black Knight; Sword of Rage
Jurassic Park Pro/Premium/LE (Stern)
Elvira's House Of Horrors Premium/LE/Signature
Stranger Things Pro/Premium/LE
Teenage Mutant Ninja Turtles Pro/Premium/LE