Listen at the volume that is most comfortable for you. Some people like all their imagery set to a very soft volume, because they want to fall asleep listening. And since one of the titles you’re using is the one for restful sleep, you probably want it low enough to let you drop off to sleep and stay that way.
The other titles you have are probably good in the conversational range, but again, you can experiment to see what volume is most pleasant, relaxing and comfortable for you.
Of course some people are hard of hearing, so they are going to turn it up quite a bit.
Some people like using head phones so they can hear everything very acutely (and without disturbing others). It all works, and there’s no one right answer, so it's completely up to you.
Thanks for the question and all best wishes,