The audio is from the CD “ボイスコレクションCD「本気(マジ)になっちゃうぜ!? 山口勝平」” (Voice Collection CD “I’m getting serious (for real)!? Yamaguchi Kappei”)
Just a clarification, this is NOT official. This is just something Yamaguchi Kappei (voice actor of Kaito and Shinichi) had volunteered to do himself, outside of voicing DC.
Also, the original video was created by an online user named 星天飞舞:
http://www.nicovideo.jp/watch/sm23825666
I only added the subtitles (my old channel name was TT)
MAG.MOE - The MAG, The MOE.