how to search a particular song using search button
You have to add Music to the Searchable items in the Search settings.

These settings you normally find when pressing the phone's Search button (to view the Search widget) -> [Menu] -> Search settings -> Searchable items and you select the items you want to include in your search.

On a HTC phone I think you find these settings here:
[Menu] (while on a Home screen) -> Settings -> Search -> Searchable items.