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Ah, figured it would turn out to be something like this. Thanks for the explanation.

That sound is part of the explosion sound. Any attempt to make it not play would in some way cripple the sound system and harm other mods so 'fixing' this does not sound like a good idea.

This effect depends on very specific properties of Doom's original sound system which ZDoom no longer has.

Played [url=http://www.doomworld.com/idgames/index.php?id=288]this venerable '94 WAD[/url] on ZDoom recently. There's a sound clip that is supposed to play during the intermission screen as part of the % tally - a long "thank you for visiting Yak World" spiel - but in ZDoom, it's appended to every rocket and barrel explosion (and every explosion during the kill tally screen). In other engines (tested in Choco Doom and Eternity) this behavior is not present; it only plays once, after the first tally explosion (and never during normal gameplay, that I could find).