Step 1: Free download Tansee iPhone Transfer and then install it. You can konw more about it by referring here. You also need iTunes above 7.3 installed.
Step 2: Connect iPhone to your computer.
Step 3: Launch Tansee iPhone Transfer and all the content of your iPhone , including music and videos will display automatically, screenshot as follows:

Step 4: Select the files to be transferred to your computer by click them. Ctrl key for multi-files select. Click right mouse button or click "file" can select all songs.
Note: you can only select first 5 songs if you haven't purchase Tansee iPhone Transfer.
Step 5: Click "Copy" button to select output path and start to transfer songs or videos to your computer:

Tips: How to Transfer Song from iPhone to iTunes Music Library?
Step 1: Transfer your songs / videos to a folder on your computer by Tansee iPhone Transfer, e.g. "C:/music" .
Step 2: Run iTunes, click "File", click "Add Files to Library..." or "Add Folder to Library..." , select the songs in folder C:/music.
Or simply run iTunes, Drag your files in "C:/music" to your iTunes music Library.