Koe De Oshigoto

Seeing as how this section has been dead for months I thought I'd make a recommendation. Anyways a synopsis:

From DesuDesu
"It’s about a high school girl whose elder sister works in an eroge company, and said high school girl, through some magical plot device, is forced to work as an eroge voice actor for her sister. Hilarity ensues (oh come on, you know it’s fun to see an innocent high school girl say “penis” and die from embarrassment after that)."

As for a sample, well here's a highly plausible way they get that sucking sound in VNs.

Anyways, you can read it at mangatoshokan: http://mangatoshokan.com/series/koedeoshigoto