S e i y u u ;
The seiyuu (=voice actor) for the English version of Mugetsu is Bob Marx, a man who's already "starred" in several anime films and shows. For example, he's the voice behind Jinnai, a bad guy from the anime El Hazard, who is known for his insane laughter. I guess Mr. Marx is specialized in that. :)
No matter how hard I tried to look, there's no info on Bob Marx available anywhere. I'll keep on looking, but until then I'll just have to stick this some sort of a cinematography here. If you know what other stuff he's been in, please let me know.

El Hazard - Jinnai
Perfect Blue - Mimaniac
Bushido Blade 2 - Katze
Magical Princess Gigi - Fuzzy the Dragon Master
Saber Marionette J - Hanagata

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The voice of the Japanese Mugetsu is Yuji Ueda. His list is a little longer, and there's more info about him available, too. Ueda-san was born on june 15th 1967. In addition to being a seiyuu, he works for Art Vision. Again, you're free to complement the list below.

Digi Charat - Abarenbou
Fushigi Yuugi - Amiboshi & Suboshi
Xenogears - Bartolomei Fatima
Pokémon - Brock/Takeshi
Oh My Goddess! - Keiichi
Hand Maid May - Nanbara Koutarou
Rurouni Kenshin - Sagara Sanosuke
Rival Schools - Shoma Sawamura
Love Hina - Urashima Keitarou
Fullmetal Alchemist - Zolf J. Kimblee