asterisk users - Jun 2005 - Programinng Aplication with Music on Hold

Hi,

I am programing  one aplications to hear my e-mail on PBX asterisk. I
want to have music on hold when the my aplications get the e-mail the 
mail server real.
I am doing un IVR to this aplicationes.

How I can do that?
When is the routines on the source asterisk  to music on hold?
What files I can see, for one example?


Thank,

Raul Pineda