Well Anthony, put this in the General Mapping Stuff, it may be useful for much mappers :D... I?ll try to help you a little bit here and a little bit there (I mean, I?ll tell you the basics here but post this in the main forum, I?ll attract more attention).
You can have speeches in the game using an entity called scripted_sentence. This entity uses original Half Life recorded speeches (phrases in general) and you can assing any of them to any character building a conversation with a little timing.
One cool thing is that you can actually create your own speeches for the characters recording them yourself. It?s easy to do, I?ll tell you how or give some link in the main forum ;).
To see all the speeches (to hear the speeches, I mean) you?ll need a program like Qped to open the PAK file (don?t worry, a file containing most media stuff of the game) and there you will find the sounds and you will be able to hear them.