Are you trying to view your music on your iPhone, or your computer? Do you want to specifically see the songs that were synced to the iPhone when you synced up?

I agree with Breefield: I think we need some more explanation.
If I take your question literally, then all you have to do is click the link called “Music” under Library at the top of the left-hand side bar.
However, if you are asking what I think you asking, then you are trying to see the music that is on the iPhone.
In the same side-bar, the name of your iPhone should appear with a little arrow near it. Click the arrow and a menu will drop down, allowing you to view your music on the iPhone and all your playlists, including videos and TV, you might have on the iPhone.