Hi all.
As I mentioned in this thread, I'm having the exact same problem. Unfortunately, it seems I have no way to change the notification volume for apps such as Whatsapp as they have the default notification sounds only within the app itself and I don't think it's possible on iPhone to change it to a custom ringtone (if anyone knows how to do this, I'll pay you with part of my soul for such knowledge! XD) However, I seem to have found a solution for email notifications/texts etc. What I've done is got hold of an mp3 apple notification tone online (or use whatever sound you'd like), imported it to Garageband, maxed the volume and exported it. I've done that a couple of times to get to the desired sound level. Then, the last time export it from Garageband, set it as a notification sound when prompted, then select it for your email/text notification sound and, hey, alerts that are audible!
I find it utterly ridiculous that Apple has not managed to fix this issue yet. I bought the iPhone 13 Pro Max outright and, as I'm sure you're aware, they are not cheap! As I have 2 jobs and one is running an Air bnb accommodation, I have actually missed important emails/messages making an impact on my income! This is my first iPhone and if I'm being honest, I love the phone... Apart from this serious issue! It's a "notification" alert, it's supposed to "notify" you! Come on Apple, please fix this ASAP. I shouldn't have had to find a fix myself and it's not perfect, but it means me purchasing your phone isn't costing me money now from missed business opportunities. I still miss Whatsapp messages, but that looks like it's something I'm going to have to deal with as it seems not too much is being done about this problem.
One last point, what are people who are slightly hard of hearing supposed to do? Totally unacceptable and it's causing a LOT of grief to iPhone owners.
Please, please, please Apple fix!