Overdrive app: how do you increase audio book volume?

iOS & iPadOS

Hi all,
I have a small problem with audio book volume on the Overdrive app. I hope someone can give me a solution.
Some of the books I get from my local library and from the Royal National Institute of the Blind (RNIB) have their volumes set too low. I tried to got to settings to increase the volume but there is a problem. The settings control in the main menu does not include a volume setting. Also, when in the audio book screen, settings (along with all other controls) such as bookshelf, account etc. do not work when double tapped.
I can use the main volume controls on my IPhone 6 but that not only increases the audio book volume in Overdrive but also the main IPhone volume and it sometimes hurts my ears!
I did get a partial solution by decreasing the main IPhone volume, going to the Audible app, increasing the volume to full and then increasing the main IPhone volume again: it did increase the Overdrive audio book volume a bit. However, I need a solution that works easily within the Overdrive app and not by using several controls to get the desired volume.
Staff at the RNIB suggested that I use the rotar to set the volume higher but again, that only increases the main IPhone volume and not that of the Overdrive app.
Perhaps it is a problem with the Overdrive app: I hope Overdrive INC staff are listening.