I mostly walk on the mild side, but I do sound for sexual pleasure; not now frequently because I'm in a good sexual relationship. I have not (yet) had the courage to ask her to sound me during a HJ but she is willing and adept at teasing my urethral opening with her rather tiny little-fingertip about to the depth of her well trimmed nail cuticle.
Sounding in the safe-zone above the curve was exciting when it was new but became commonplace and contributed little to arousal and orgasm. It came alive when I got a Sims sound and reached the hot zone (prostate/sphincter area). To do this, you have to insert the sound before erection comes on. When it does, it will grip the sound but not prevent its removal or cause pain. I do not intentionally sound into the bladder and do not wish to and risk damage to the sphincter by doing so.
When you insert the sound, you will know when you have reached the hot zone. Tiny stroking movements with the sound will stimulate erection instantly and intense orgasm/ejaculation come on even more quickly than the sensation would suggest. I continue to stroke to the "point of orgasmic inevitability", the first (precum) contraction, which is usually about a second before the several that are the cumshot series. I slide the sound out slowly, to get it away from the ducts at the prostate, until the series starts, and then slide it out quickly. Extracting the sound leaves the urethra open behind it, which may take a contraction or two to fill.
This is a sophisticated form of masturbation and takes calmness a gentle hand. One who "loses it" during orgasm should avoid doing this because the endorphins will block pain and may result in accidental perforation. But done right, with attention to cleanliness, can be pretty rewarding.
I do not wish stretching or mutilation, so I am limited to about 3/16"/5mm. My sense is that a ribbed sound of that size could be invigorating. So might rigidly attaching it to the post of an ultrasonic toothbrush driver, which I plan to do. I'd like to see Japanese technology brought to bear on this.
Like riding a horse or a motorcycle, you have to get into the right mood to do this. Its better not to try this if you haven't ejaculated in a while, because the urgency can make you careless. Doing this should be treated like taking a hot bath...take your time to pamper yourself and pay close attention to the sensations.