In Legends,C-3PX was a Cybot Galactica 3PX-series protocol droid reprogrammed to be an assassin droid. 

Maul came upon the droid at 33 BBY. Darth Maul, not entirely interested in the prospect of having a partner, much less a droid partner, originally planned to utilize the protocol unit simply as a method of dealing with distractions such as customs stops, port authorities, and other things Maul preferred not to deal with. 

However, after some consideration, Maul discovered that, with some extensive modifications, C-3PX could become a competent droid ally and a form of protection or a way to spread his influence. Using this plan, Maul completely rebuilt the droid, installing defensive lasers, stronger droid plating, and other protective measures. While an exterior view revealed nothing more than a reassembled chassis and plating resembling a 3PO-series protocol droid, eighty-three weapons were concealed within the droid's recesses in its joints and arms. 

The Sith apprentice also reprogrammed the droid to only bother Maul when an important matter came up, or if an issue was reported in one of the starship's systems; as such, the droid stayed mainly silent under the Sith apprentice. During his missions, Maul would leave the droid aboard his heavily modified starship, confident in C-3PX's abilities in guarding the transport.

C-3PX was a cold, calculating droid, who dispatched those he was commanded to kill easily. While serving under Darth Maul, the droid spent most of his time in silence, due to Maul's modifications to his programming. Not much was known about his personality after his memory wipe and service under Raith Sienar, his company Republic Sienar Systems, and later the Grand Army of the Republic; however, as his mission profile dealt mainly with his ability to go about unnoticed, it might have been such that C-3PX shared many personality characteristics with other protocol droids of his day.

Ideally, C-3PX should sound like a slightly eviler C-3PO, or you can feel free to use your imagination or whatever you think her should sound like.