I know a couple of women who are into this. I've got a set of double ended Hegar sounds (they're actually Cervical Dilators, but they work well). It kind of depends on the woman. Some women have hated it. Some women I've used them with have gone into orgasmic convulsions just by sliding them in. Most of the ones who have liked it, have liked a vibrator on their clit while the sound is about 2 inches in, and being gently tapped on to give it a bit of a rocking motion (up and down).
I'd suggest you get a set of sounds, and play with your girlfriend. They'll cost you between $70 and $300 depending on where you get them from (A link to a site that sells them is below), but since your gf cums the way she does, it will be well worth the investment. Use a LOT of lube and start with the small size, and work your way up. Go slow, and let her tell you what feels best.
They payoff for you once she gets done cumming should be amazing.
URINARY SOUNDS