I must admit I've never heard this actual term, and not seen the equivalent in EN either — but as a former theatrical sound technicaal, I can understand perfectly what they mean: when the auditorium is particularly deep, additional sound reinforcement speakers may be needed say 2/3 of the way back; but the sound going to them travels faster than the sound travelling through the air, meaning that people sitting further back in the auditorium hear an objectionable pre-echo from the speakers first, and then from the stage. To overcome this, a delay (sometimes several progressive delays) are introduced into the audio feeds, longer and longer the further back you go in the auditorium, so that the sound coming out of the side speakers comes out at exactly the same moment as the sound wave passing direct through the air arrives.

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Note added at 59 mins (2008-11-24 16:28:12 GMT) Post-grading
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Been there, done that...got the earache!

As for 'plane' of course, this would be referring to the notional 'plane' in which lie all the loudspeakers operating at one given sound delay; so a long auditorium might have several 'planes' containing speakers at progressively longer delays.