Automatic audio source selector

The circuit selects automatically which audio source to use and turns on the amplifier. Based on automatic switch I have made earlier: http://www.electrobob.com/auto-amp/ Details: I need a 4 to 1 audio selector, so my amplifier can use which of the sources is available(PC, tablet, bluetooth, raspi). Simple relay switch and not a mixer, since some of the audio sources are "clean" some are "dirty".