(the comma in the spoken sentence kind of rids it of it's urgency as well - its unnecessary really)
Instead try something like;

"Try and hold back as many as you can while we make it up the stairs!" Ian screamed into the radio, to the lone sniper who waited on the roof. The radio craclekd with an inaudible voice, then the sound of gunfire echoed through the hall.

And one more thing (I realise this post may be a little hard to follow, I'm just mentioning things as I scan through it) why is the sniper "cold and emotionless"? I realise this may be the personality you want to give her, and would fit with a sniper I suppose, but it's a little intimate a statement of her personality when she hasnt even had a single action/line in the story apart from implied behaviour (e.g. shooting the zombies - which hardly amounts to emotionless). If you want to make her seem cold, you could do;
"Jesus, what the hells going on..." I muttered "I know some of those people out there. What the hell's wrong with them?!" a note of panic entered my voice as I recognised the monsters below.
The sniper merely fired another shot at the crowd, before reloading her weapon. Whilst she did this she deigned to answer me, in a cold, unemotional voice.
"If you want to survive, you have to realise those people out there are not people anymore. They are not your friends, or even your family. They're nothing but things".

But this is a very, very very well written story by this forum's standards, and even outside of that it's still good, with decent character interaction. I suppose to continue the story you could have them escape from this building to the next (jump from the roof top to roof top???), or use the fire escape to escape to an alley with a connection to the sewers, or a helicopter could rescue them (after a prolonged siege of course), or they could all die horribly.