We can increase the volume on our Nest Hubs as described in the link below, but there's no way I know of to get a higher volume just for a "visitor announcement".
Thanks again for helping here, @MplsCustomer. @Workdammit, I hope you've got the answer you're looking for. About the volume of your Nest Hub 2nd Gen and Nest Doorbell notification, I'm sorry to say that there is no such option to change the volume for notification. You can change the volume of your Nest Hub itself.
We appreciate people who are keen on sharing their thoughts with us. We're always looking for ways to improve and we'll take this as feedback. Let me know if you have other questions or concerns, otherwise I’ll be locking this thread after 24 hours.