Why can't she take it off? She can even use it as a projectile weapon.

And then she'd expose her head.
Is the trade off really that worth it?

No one goes for the head in Power Rangers—no one.

I'd think if she used her Sonic Scream in her helmet the echo and reverbiration of the sonic sound waves would bounce around, amplify in the enclosed space, and rupture her eadrums if not also do massive trauma to the soft tissue. Ears/Nose/Brain/Eyes. Conner and Ethan would have easier times using their powers in suit, but Kira would almost certainly kill herself if she tried.

She'd shatter her visor before all that would happen (Outside the eardrum thing), but yeah. She can't use it while helmeted.

But I'm fairly certain Gia has some kind of card to make her regret pulling off her helmet should she try.

Bad Hair Day Dynamic!

Kira using her scream in-helmet would just be suicide either way. It gives Gia an opening, and her ass on a platter.

You can breathe on the Moon in the Power Rangers universe, in addition to the fact that you can hear in space. Sound does not work the same way in that universe as it does in ours. However, you do bring up a good point, but if Kira were to remove her helmet and scream at Gia, the momentary pause as Gia clutches her ears in agony will give Kira enough time to repeatedly bash Gia with her helmet in a (fairly comical) pre-finisher, after which she will initiate Super Dino Mode and blow Gia to smithereens.

>Super Dino Mode

lol. She had the most useless one.

Hers gave her wings that were so thin you could read a newspaper through it.

All of the Super Dino Modes granted the same powers. Kira actually received the better upgrade (mathematically speaking), since she received wings with which she could fly in addition to the boost in strength.