David
Sorry about the delay in responding.
There are a couple of audio volume settings on an iPhone. I wonder if that is the issue.
On an iPhone the volume of the notification sound is controlled by the “Ringer and Alerts” level (see >Settings> Sounds & Haptics). There is a switch below this setting “Change with Buttons” to allow this to be adjusted with the buttons on the side of the phone)
Have you had a look at:
https://maprunners.weebly.com/phone-settings.html ?
Maybe try a custom sound and see if that helps.
(I'm using MapRun version 7 on my iPhone regularly and am getting sounds)
Peter