they're used for probing and dilating passages within the body. The most common are the urethral sounds that we're discussing. I'm guessing they're called sounds because of that other instrument that you ting and it's makes that vibrating noise, they're made of the same material. also, in the "good old days" before penicillin, gonorrhea was much more difficult to cure and many times left scar tissue behind, causing "strictures" which partially or completely closed off the urethra, making urination difficult if not impossible. Some benevolent medicos (Drs. VanBuren and Dittel, to name two), seeking methods to relieve this problem, designed instruments to be inserted into the urethra to locate the stricture and literally stretch it open to allow the passage of urine. Thus was born the urethral "sound" which takes it name from the Latin subordinate, to submerge to measure the depth of. This is the same "sound" as used in the term "sound the depths," where the depth of a body of water is measured with a weighted line.
http://www.goaskalice.columbia.edu/3516.html
http://www.bodymod.org/forum/printer...s.asp?TID=4829