In the character file, you can assign the voice clip which is spoken when the dialogue starts in the line ch.greeting. In RESOURCE\INI\ALIASES\Greetings_alias, you assign one or several clips to that greeting. If it's more than one, the game will choose one randomly.
You can also add a "play sound" to any dialogue case in the dialogue c-file. For an example, look at the dialogue file of Lil Carlos (Devlin Opera). There's always a construction sound when you talm to him. JRH uses this quite a lot to let characters make sounds or say things during dialogues.
I don't know if your clips might be a bit too long. Not from a technical standpoint, it should work anyway, but just from a "logical" standpoint a twenty second voice clip might be a bit too much.
Thanks!
For some reason when trying to do PlaySound("VOICE\ENGLISH\DavyJones1.wav"); nohing happened, and after trying to launch the game again it crashed... i will try it again.