Modding Help Moontant Monster Type Doesn't Play Sounds From Active Effect

So, I made a gun that applies an effect to a monster hit.

The effect lasts one second.
if the target has less than 0 health and we haven't already played the sound, then:
Play the sound, Send to log "All clean again", Set self.doneTheThing to true.

Code:

function init()
    self.doneTheThing = false
end

function update(dt)
    if status.resource("health") <= 0 and not self.doneTheThing then
        animator.playSound("clean")
        sb.logInfo("All clean again")
        self.doneTheThing = true -- We did the thing. Don't do it again.
    end
end

function uninit()
end

This works for all entities, including players, apart from moontants (the erchius human things from the Erchius Mining thing mission).
It sends "All clean again" to the log, so I know that it's applying the effect, and that it's passing the check, but the sound simply isn't playing, or cannot be heard.

Does anyone have any ideas at all as to why the sound isn't playing or can't be heard for this specific monster?