I want to use this mod's alarm to play some sound such as "Dear Passengers, Our train has......".

Can it work if I only add some .ogg files in assets and edit the sound.json ?
If not, where should I edit?

Sorry for my poor English and look forward to your kind reply

There's a nice guide here: http://wiki.feed-the-beast.com/Howler_Alarm :)
It's not really necessary (or recommended) to fork it, you don't need to change any code :)