There's a mistranslation around 7:30:
Yonghwa doesn't say that it "echos." In Korean, "to echo (wullida)" sounds similar to "to (allow someone to) hear (dullida)," so they might have misheard him.
He says "Because this is outdoors, (everyone) can hear it. So we couldn't (rehearse) a lot, just as much as we could. It's so awesome, the sound."
Since he loved the sound quality, I don't think there was an issue with the echo to begin with....