"helpless voice" is correct and usable in written English.
You can use it to refer to someone who has a quiet and powerless air about them when speaking. For example, "He looked up with a helpless voice and asked for help."
Again and again, we see Eli lure adults to their doom by crying out in a helpless, little-girl voice, "Help me; please, help me!" only to pounce when they come within biting distance.
Yet our most important first alert to the suffering of a helpless child is the voice of witnesses.
Her voice sounded helpless and small, like a chipmunk with an injured leg, rescued and nursed back to health by an old man who discovered her on the side of the road — a man who spent six months trying desperately to teach that chipmunk to speak so that he wouldn't feel so alone anymore, until finally, on his birthday, of all days, he woke to a voice at the foot of his bed.
Her voice sounded helpless and small, like a chipmunk with an injured leg, rescued and nursed back to health by an old man who discovered her on the side of the road a man who spent six months trying desperately to teach that chipmunk to speak so that he wouldn't feel so alone anymore, until finally, on his birthday, of all days, he woke to a voice at the foot of his bed.
Because it gives to the narrative voice a numbed, helpless quality.
"Help," I cried, hearing my voice and how helpless I sounded, wondering if this would be the last word I would ever utter.
They described him as charitable, kind, blessed with a beautiful singing voice, and compassionate toward helpless people.
Class suits were supposed to give a voice to victims too helpless and dispersed to get justice on their own.
We felt helpless and looked to find a voice".
