We’re all listening to music on our phones and some of us are even using our phones to play back music over a PA system (preshow music, sound cues, etc…) I was looking for the output impedance specs of an Iphone and came across this measurement information related to the Iphone.

archimago.blogspot.com – MEASUREMENTS: Apple iPhone 4 & iPhone 6 audio output
kenrockwell.com – Iphone audio quality
stereophile.com – Apple Ipod measurements