How do I get sound out of galaxy 4 to come through Aux port in my car? I have a Toyota Sienna.  I am trying to get an audio book located on my Galaxy S4 to play through the Car's sound system.  I attach the cable to the Aux port in my car and turn on my Galaxy 4 audio book. The Sound comes out of the phone but I can't get it to come out of car's speaker system.  When I plug in my iPod tothe Aux port, the music plays just fine in the car's speaker system.  What am I doing wrong?
Check that the app you are using to play the e-book is correctly outputting the sound to the aux-cable (in case it has some setting to disable/enable audio output to headphones). Alternatively, try some other apps, to check sound output through the phone, to the aux cable, into the aux port and to the car music system. Also, as @Zeiss Ikon suggests check sound output from your S4 to a pair of headphones, external speakers etc.

Most of the times, the volume is also reduced for headphones and Android warns on possible ear damage. So also try increasing the volume.

Try with a different aux cable. Though if the iPod is being connected with the aux cable and it is working, theoretically it should work.