if i want to write a plugin to support other audio file (not mp3),
anyone who can tell me how to write a amaroK plugin?

That would not be a amarok plugin. amarok uses other engines to play mp3. amarok is able to use these engines. To add support for a filetype you should add it to one of those engines. And you would probarly have to allert the amarok enginers so they can change amarok to add that filetype to the database.
But most probarly the filetype you are trying to play is supported by one of the engines, so you should realy find out if that is the case and then only change to that engine (in Configure - Engine).

We wouldn\'t have to be alerted. amaroK decides whether or not it can play a file based on what the engine tells it.