No, that's not a typo for ethereal sound, but the name of a medical instrument (and sometime sex toy) for dilating the urethra. Now, even though I am a fan of the animated series King of the Hill one of whose plot points is the character Hank's narrow urethra (almost always whispered), I'd never heard of these instruments or their extra-medical usage. (oh, the sheltered life of a philologist.)

The etymology of this kind of sound (not the vibrating air or body of water) traces back to the French sonde 'sounding line'. Urethra goes back to Greek ourethra < ourein 'to urinate', cf. Latin urino.