The Mute Button Won't Mute AVAudioPlayer

I realized it was because I had this line of code

[[AVAudioSession sharedInstance] setCategory: AVAudioSessionCategoryPlayback error: nil];

I then changed it into

[[AVAudioSession sharedInstance] setCategory: AVAudioSessionCategoryAmbient error: nil];

Then the mute button would work and stop my AVAudioPlayer...