Good to hear about the alternative.
After I wrote about Power Amp, I tested it further and found it did not need my needs.
It is fine and ear shattering for music or any audio from any folder, when played within itself, - but it doesn't increase the volume when watching a movie or video from Gallery.
And whilst I am sure there is a way of stopping it, it seemed to play everything within a folder one after the other, with seemingly no way of stopping it.
Perhaps one has to make special play lists.
But I found it frustrating and not very easy to control, - particularly to stop. It just would not stop playing things within the nominated folder.
Anyhow, it has now been deleted.
I particularly need some app. which will boost volume from Gallery played videos and movies.