Figured I might as well post this up, since I've seen a bunch of threads on this matter in the new CK section of the Skyrim Nexus. These two tutorials are invaluable, and contains just about everything you need to know to get a custom-voiced companion into the game. The third link goes to the Voice Actors of Skyrim thread, which is also very helpful in finding appropriate VAs for your project. Here, take a look:

http://deck16.net/post/22645519500/making-a-unique-voiced-follower-in-skyrim-part-1

http://skyrimmw.weebly.com/skyrim-modding/making-a-custom-follower-skyrim-modding-tutorial

https://forums.nexusmods.com/index.php?/topic/951656-the-voice-actors-of-skyrim-nexus/

Oh, and if you ever need a framework/ inspiration to write quests/ dialogue for your Skyrim mods, then this link contains everything I've written for my Skyrim projects thus far. Feel free to take a look:

https://drive.google.com/open?id=0B62TMcfDg13cY01EQ1FJZDlsazg