Hello FiSaxon,
Thanks for the post! Audio notifications are a great function, and I understand you're not hearing them for some types of notifications in WhatsApp. I'm glad to see what help I can offer.
It sounds like you checked Settings > Notifications > WhatsApp already. Is that right? Use notifications on your iPhone, iPad, and iPod touch. Have you also checked Settings > WhatsApp to see if there may be any relevant settings there?
It might also help to make sure your iphone and the app are updated:
How to back up your iPhone, iPad, and iPod touch
Update your iPhone, iPad, or iPod touch
Update apps or use automatic downloads
If that doesn't seem to clear things up, you could reinstall the app as well: How to delete apps on your iPhone, iPad, and iPod touch
If the issue persists after, I recommend contacting the developer: How to contact an app developer
Take care.