In the very beginning of the movie, right before Leeloo punches through the glass that is supposed to be "unbreakable", you can see that the glass is already cracked and ready to be broken.
It also helps to watch the acting and see that LELU opens her mouth as if making a noise, the camera focuses on her face through the glass. Before she opens her mouth the glass is intact, after she opens her mouth the glass is broken.
Conclusion? They want us to think that LELU can shatter the glass by sound.
Nah... I tend to agree with the original commented slip-up. If in fact Leelo was able to break glass with her voice, the entire tube would have been cracked. It just appears to be plan and simple "Milla's not strong enough to break through Plexiglass".
It's hard to say with this one. The glass was cracked just before Lelu punched through it. But it looked like she was growling at the general, like a threat. She wouldn't really know that it was unbreakable. She certainly didn't try very hard to break out before he started talking to her.
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