To transfer all music/videos from your iPhone to your computer and iTunes, all you need is to use this handy program called Tansee iPhone Transfer

http://www.gutensoft.com/Utilities/Tansee-iPhone-Transfer.htm

Many of my friends are using it for its easy and safe transfer. The whole process goes as follows:

Firstly install this program on your computer. When you connect your iPhone to the computer, this program would instantly display all music and videos on your iPhone. All you need is to select what you wish to transfer and click "Copy" button. Done. All your selected files will be copied to your selceted folder on the computer safely. Now you can easily add those files to the iTunes library.
More importantly, this won't overwrite any original files on your iPhone.

Here is step-by-step guide on the whole process:
http://www.gutensoft.com/Guide/How-to-transfer-songs-and-videos-from-iphone-to-computer.htm

Hope this can help.
You can also search more others at http://google.com
But I like to use this one best.