Sounding or urethral sounding is the medical use of probes called sound

Sound (medical instrument)

In medicine, sounds are instruments for probing and dilating passages within the body, the best-known examples of which are urethral sounds and uterine sounds. Some people enjoy using them for urethral play.-Urethral sounds:...

s to increase the inner diameter of the urethra

Urethra

In anatomy, the urethra is a tube that connects the urinary bladder to the genitals for the removal of fluids out of the body. In males, the urethra travels through the penis, and carries semen as well as urine...

 and to locate obstructions in it. Sounds are also used to stretch the urethra in order to receive piercing.

Urethral sounding and urethral play are also used to refer to this practice in a sexual context, mostly due to physical problems with the genitals (because the hymen is too hard or because of the absence of the vaginal opening).

Urethral play can involve the introduction of either soft or rigid items into the meatus

Urinary meatus

The urinary meatus is an orifice of the urethra. It is the point at which urine and, in males, semen exits the urethra. The meatus features varying degrees of sensitivity to the touch among males and females....

 of the penis (as well as farther in).