Loud notification sounds

Honestly, this is very specific to each user. I think they're all loud, but you can adjust the volume for these:

-> Use tones and ringtones with your iPhone, iPad, or iPod touch - Apple Support

If you are having difficulty hearing them, have you tried enabling the option to Repeat the alert? For instance, if a message goes unread, the notification sound will chime again until you read the alert. It can Repeat Once, Twice, 3, 5, or 10 times.

Go to Settings > Tap on Notifications > Scroll down to Messages > At the bottom select "Customize Notifications" > Select "Repeat Alerts" and choose your preferred repeat time.