[quote=Silver Carrot]
The scream, I'm using, but I want all her powers to come from her own voice. She could maybe mask someone's footsteps by mimicking a loud gust of wind that seemed to originate from an open window, but she couldn't actually affect noise caused by anyone else.
[/quote]

Ah, but she would be using her own voice :)

You know how sound is a wave? Well, if you were to put another wave alongside it but out of phase, then you effectively cancel it out. So what I meant was not that she could directly manipulate sound, but that she could generate sound waves that negate others. She would mask someone's footsteps by generating the out-of-phase sound of the footsteps, effectively canceling them out! All with her own voice!

Don't believe me? Search noise cancellation on wikipedia ;)

The limit for it would be that she could only really do it for one particular area at a time..

The boosting on the otherhand.. Thats exactly the same principle only that instead of being out of phase, it would be [i]in[/i] phase. This would also only work for one particular area at a time, but that area would be bigger because it doesn't matter who hears it :D