Koji Igarashi and the Castlevania crew in 2008 took to the airwaves via the Internet and radio. They undertook two projects starting with Akumajou Dracula: Radio Chronicle series, which ran for a couple of months and featured 13 volumes. The series' content had little relation to the video games and was described to me as "tedious." Akumajou Dracula: Radio Chronicle was released on CD in 2010, available only on Konami's online store; scans of this product can be found here.
The other project was the Internet audio drama Akumajou Dracula X: Tsuioku no Yasoukyoku (translation: Demon Castle Dracula X: Nocturne of Reminiscence), a 6-volume special that chronicled events a year following Symphony of the Night. While I'm told that this drama, too, was "tedious" and not worth translating, it had a much broader frame of reference. From what I gather, the storyline unfolds when several young women are killed by what seems to be a vampire attack, and Alucard somehow becomes the main suspect. It becomes Richter's mission to discover the truth. Other characters are of course involved.
This complete radio drama, including a newly created Volume 7, was also released on CD in 2010. It, too, is currently available only at konamistyle.jp. To view the product's packaging scans, click here. Furthermore, Japanese customers can earn a special bonus for purchasing in one order Radio Chronicle, Tsuioku no Yasoukyoku, and the Castlevania: The Adventure Rebirth soundtrack. (All scans and information were provided by JPCVFAN.)
