First off, make sure that your phone isn't muted or set to silent mode. Then try to see if the speakers on your phone are working; just stream YouTube videos and play it on the speakers to see if your can hear some sounds. Aside from that, make sure that your phone isn't paired with any Bluetooth device because it's possible that the audio is channeled through the Bluetooth device. Moreover, make sure that your custom conversation notifications are actually playing because you might be using music files that actually don't play.