your adding or replacing the civ in your mod ?
you can change the speech of an existing civs with a MP3 (vfs set to true) with the name of the original DoM speech for that civ.
as an example, even if I've renamed Russia to URSS in my WW2 mod (WIP), the civ is still referenced as "CIVILIZATION_RUSSIA" in the XML and I've put an audio MP3 of Stalin named russia.mp3 in my mod, set the VFS to true for that file, and voilà...
I've done the same for France.mp3, England.mp3, Germany.mp3 and Rome.mp3...
for a complete new civs, I'm not sure if it could be done.