How to turn off iphone speakers/sounds

My husband is deaf. He hears nothing. So far we’ve been unable to turn off the sound, to turn off the speakers on his iPhone 15. Whenever he communicates with other deaf people in FaceTime or other video relay apps we are unable to shut the sound off by sliding the sound slider down to the bottom. It always pops back up again. This is an issue because I hear just fine and some of his friends are pretty loud or they have the TV on very loud. I just want to figure out how to turn off the sound on his iPhone. Previous iPhones we would plug-in a headphone to a speaker jack, but there are no more speaker outlets in the latest models. Thanks for any help.