Apple iPhone 4 Dock - Manual (Page 5)

English 5
To play music through external powered speakers:
1 Using an audio cable that has a standard 3.5 millimeter
stereo miniplug, connect the speakers to the Line Out port
on the back of the dock.
2 Put iPhone in the dock, and then play a song.
For complete information about using iPhone, download the
iPhone User Guide at www.apple.com/support/manuals.