How do you call your sound (the entire line of script, not the command)?

Nope

That's an OPEX behavior, I imagine.   You could try that (no guarantee): in initPlayerlocal.sqf , recommended, (or where player is already defined):   player addEventHandler ["GetInMan", {   params ["_unit", "_role", "_vehicle", "_turret"];   _unit spawn {sleep 1; _this setCaptive FALSE}; }];  

Slick.  Good job NumbNuts!

or in marker area (you can place a marker area and set it invisible (editor or _marker setMarkerAlpha 0) si you don't want to see it on map:
waitUntil {sleep 1; {_x inArea _marker} count allMines >= 20};