A sounding board is someone with whom you can try out an idea or practice making an argument. The reaction of your sounding board gives you an idea of how your eventual audience will react.
A writer might have a good friend who acts as a sounding board, reading drafts of her novel before she submits it to a publisher. Big companies also use sounding boards, trying out new products or ideas on them to gauge how successful they might be. The original, literal sounding board is a device — often an actual board that vibrates — meant to help amplify a speaker's voice from a stage, podium, or pulpit.