Answer to: no notification sound on text messagesMichelle, a forced stop of Hangouts (or any app) is only good until you reboot, which will auto-restart the app. I recommend that you change the Settings in the Hangouts app to prevent your SMS texts from being redirected to Hangouts. I'm not a Pro, but it's what I experienced.
Answer to: no notification sound on text messagesHANGOUTS settings will override SMS Messaging settings. My daughters new (Verizon) Galaxy S4 Mini would only count one incoming SMS text after a reboot on the Home screen, while 2+ unread text messages were waiting within the SMS app. Go into Hangouts> Settings and deselect the checkbox that redirects SMS messages to Hangouts. Be sure to go into the SMS>Settings and enable Notifications. This solved the problem for us. Best of luck:)