I'm compiling a list of places in Anglo-Saxon literature where devils, demons or the Devil himself speak. Off the top of my head I have:
Genesis B -- the Devil in that whole Fall of the Angels and then Fall of Man thingy.
Juliana -- the devil that Juliana captures and forces to confess.
Christ and Satan -- the Devil.
Guthlac -- the devils who torment St. Guthlac by showing him monks being bad (oh, and dragging him to the hellmouth and threatening to throw him in).
Andreas -- when the devil shows up to convince the men to attack Andrew and later encourages them when they are torturing the Saint.
Elene -- devil shows up to stir up the people to go against Judas. (I had forgotten this. Thanks Jason Fisher)
Are there any I've missed in the poetry? Can you think of any in the Prose?
Yes. Gospel of Nicodemus (thanks Vellum)
Also, possibly, sermons for first Sunday in Lent because the text is Matt 4:1-11 (thanks Derek).
LS 14 (MargaretCCCC 303) 16.7: se deofol hire to cwæð: Sathana urne cyning, hine gewræc drihten of paradises myrhþe (cf. Pass.Marg.[Par] 10.7 tunc demon dixit). (thanks Hilary)
And I will check out Peter Dendle's book (Thanks Dr. Virago).
Looking through Saints' Lives and using the concordance to find references to Satan and Devil will certainly be a way to go as well, but I am mainly looking for long speeches by the devil or demons rather than things about them.
Updates: Vellum points out some devil talking in prose: Gospel of Nicodemus. Jason Fisher reminds me that the devil makes an appearance in Elene as well. Derek suggest sermons for the first Sunday in Lent. Dr. Virago points to Peter Dendle's Satan book (you know he also wrote a book on zombies. How cool is that?). Eutychus points out places in Scripture that would work and Hilary notes the Life of St Margaret.