I noticed it's been aliased to urethral, and I hope that was an oversight. It makes little sense since sounding normally relates to using a rod. When people search for sounding, the catch-all term picks up things outside of it including fisting, fingering, penis-in-penis, etc.. It's not for everyone. These other tags imply penetration too, so why does this one get left out?

I believe sounding was aliased to urethral because it often used to tag any sort of sounding.
If you want to see images involving use of a urethral sound, see urethral sound