Okay, I think I found the problem. I was so concentrated on the music that I neglected to notice the orange box around the slider area, which means the slider had already received the focus or been selected. When I touched it a second time, the volume changed, which is just one way to do it.
I tested my theory again on a different song and sure enough, one touch created the focus (orange box appeared), and then the volume could be changed with the wheel, + or - buttons, or by physically sliding the volume. So, it works.
Thanks to all who read my post!