how to force  image file and music file  to download with zip format in drupal 6?
I'll answer with a question, do you want to make a zip file because it's an easy way to force browser to download the image or the mp3 instead of showing it ?

If so I'll recommend you the File Force Download module to achieve this. Also you have DownloadFile module and a .htaccess way to do this: Force Files to Download Instead of Showing Up in the Browser but I don't find this one to be very flexible.