@Jenderbys: That's strange. Can you head to Settings > Notifications > Do Not Disturb, and make sure that this option is toggled off? Now, head to Settings > Sounds and Vibration > Select 'Sound' > Notification Sounds > Assign a sound to the relevant SIM > Back > Volume > Make sure the Notification slider is at a suitable level. Then, head to Settings > Apps > Messages > Notifications > General Notifications > On > New Messages > On. If the issue persists, can you confirm if this is affecting all apps, or just the Messages app?