I have some hearing loss, and have used a cupped hand to help focus sound for many years. It can help you hear you own voice in a crowd. If you actually insert a finger, it further isolates YOUR voice from other sounds, but also reduces amplification. (I have now gotten hearing aids to do some of what I need, but many times they are TOO loud, so cupping the hand still is useful...

......however, when I was in college, there was a guy who was director of the Physical Plant (building & grounds) who was occasionally observed walking about with his pinky finger inserted in his ear, so it looked like his arm was partially supported by the 'hook'. People remarked on this for years, but as far as I know, no one ever got up the courage to ask him why...grin*..he never sang, as far as I know.(he was 'strange' in several ways...a real character)