So far it looks pretty good the power is extremely interesting. 

However her weakness of covering the mouth shouldn't do the trick if she can blow back objects with sound. If anything the two main weaknesses should be if somebody disrupts her vocal chords or if there is absence of anything for her to transmit sounds through, like being in a vacuum. Then again in that case she has more problems then not being able to use her power :P 

Her being part of the DSA is interesting but there should be a reason why a federal agency would recruit an inexperienced foreigner to join their ranks. Maybe if she was American or if she had more "tactical" experience in Britain it'd be more plausible as to why they'd want her. Because the sheet implies she was headhunted by the DSA not somebody that tried to apply. And there should be an eventual connection the League. Maybe a Liaison/Observer?