What does this mean?
I only have my 1st person controller ,terrain,camera.and light in the scene
It means that you have two audio listeners, probably one on each camera. You can only have one audio listener active at any one time. Either you have to enable/disable them as you switch between the cameras or remove one of them.
You can find both audio listeners by typing in AudioListener in the search field above the heirarchy tree view.
Click the “Main Camera” from “Hierarchy” panel and remove tick from “Audio listener” component from “Inspector” panel.
Delete the camera, the first person controller already has one on it 
I have also a question about this: I have a splitscreen game with 2 cams; 1 for each player. Now the surroundings of the 2 players have to be mixed into the output, so there must be 2 audiolisteners at both of the cams. How can this be done?
Nothing happen how many (Audio Listener) is added in your scene. This is nothing affect on your project. Just check in the (Audio Source) component the sound which you want to playing is added perfectly either this is FPS Controller footsteps sound. Check in the Audio Source component the Volume is set to 1 not to 0. Check any mistake of adding the sound in the Audio Source. Any Mute button is disabled in the Game window box.
Thanks Alot man
Unchecked One Of Them
You have multiple cameras on the scene. Each camera has an audio listener component, so you can remove or disable them except for the main camera’s audio listener (actually you only need one audio listener)