you can stretch any hole in the body, if done gradually! i'm not that familiar with sounding, but if you want to do it to stretch your urethra, take weeks to get to your goal. thing is, you will have to continue doing it to keep it there for a while. i stretched the piercing in my ear to a 4 gauge, about 3/8th of an inch. it started as a regular piercing, about a 1/32in. i took my time, over the course of months (it really could have been done quicker, but i don't like pain, nor scarring). if i take it out for a few days, i have to work it back up to that size, tho it will never go back to the original piercing size. those people with the huge lobes? that did NOT happen over night.
look it up online, there is tons of info and stuff...