I was about to say, "No, sounding isn't about prostate stimulation," but then I remember asking about these little J-shaped rods when I worked at Ye Olde Fornication Shoppe (not its actual name), and it turns out that you can stimulate a prostate by jamming gently inserting a stick in your urethra. And I was going to put that behind spoiler code for the faint of heart, because it makes even me squirm, but it turns out I don't have spoiler-usage privileges or something like that.
But, yeah, if you're wondering what exactly sounding is? If you've watched Deadwood, you've seen it in action.