This, in LemGame.pas, implements this feature (EDIT: see next message, the following isn't the best way to do it):
/// Line 6617 in SLX ///
procedure TLemmingGame.CheckAdjustSpawnInterval;
var
NewSI: Integer;
begin
if SpawnIntervalModifier = 0 then Exit;
NewSI := CurrSpawnInterval + SpawnIntervalModifier;
if CheckIfLegalSI(NewSI) then
RecordSpawnInterval(NewSI);
CueSoundEffect(SFX_ASSIGN_SKILL); <------------------- {new line here}
end;
Note that it's almost certainly worth using a gentler sound effect whilst NL doesn't support pitch-shifting. I've attached one here that I'm currently using as a placeholder in SLX whilst I hunt for a better sound. Eventually, I'm hoping to use the bass_fx.dll plugin to implement pitch-shifting up and down, but this is a way off yet. I've got lots to learn about sound library programming.
EDIT: The sound plays whilst in normal speed, but not whilst paused. The reason for this isn't apparent.