Koe de Oshigoto! is a story about Aoyagi Kanna, a 16 year-old girl who was asked to become an ero-ge seiyuu on her 16th birthday by her 28 year-old sister at her ero-ge company. Can she do the job properly while she's still innocent and inexperienced?!
(Seems Fakku dropped the series: you can find it on MangaFox.)