Further to my original question below I have received feedback on this from Luma Support.
Just to clarify the issue, this was experienced when trying to use the Voiceover feature on an iPhone 8 and an iPad Pro 11.
Luma support replied;
Yes, what you observe is by design--it's a hardware limitation that only certain iPads, and no iPhones offer volume control for this feature.
I’m guessing that if you have an external microphone connected there could possibly be controls for adjusting the input level.
Hope this is useful to others.
Is there a difference between the voiceover function when using an iPad or IPhone.
When I use an iPhone, I don’t get the option to adjust the microphone input levels.
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There should be a slider at the top!