回答
This is not a piece of advice to you I'm talking to myself for my practice of writing skills. If you would like to become a linguist I'd not be an adequate person to advice you. why does 'wouldn't' used. I'd have liked to have heard somewhat additional eg I suppose '*****n't is more suitable. I'd not be eligible to answer this question cos I can hear English on only youtube videos perhaps you might be the same situation. perhaps the native people of,Japanese or American, they might use phrases by the reason that they feel it's natural, they hear it often, their mom and papa use it. I suppose all of language usage have not always reason. even if you couldn't agree to it now I suppose you'd be good to accept it as it is for your forward step.
I was afraid that my voice wouldn't reach her.
I was afraid that my voice couldn't reach her.
I was afraid that my voice shouldn't reach her.
I was afraid that my voice mightn't reach her.
I was afraid that my voice won't reach her.
I was afraid that my voice can't reach her.
I was afraid that my voice shall not reach her.
I was afraid that my voice may not reach her.
I was afraid that my voice must not reach her.