Why does my AirPlay stream distort at high volume? I have an airport express connected to basic (but not crappy) speakers. When I run audio directly to them it plays fine, but when I run audio from either my iPhone or Mac to the speakers via AirPlay, I get crackly distorted sound if the volume slider is above about 75%.

Does anyone know why this could be? I have the airport set to extend my current network; could that affect it?
Network issues only ever result in the audio cutting out, they can't really change the content.

Distortion means your speakers get overloaded. Turning down the volume is your only option.

The reason your speakers are fine on the Mac is that after the slider in iTunes the sound gets further attenuated by the system volume buttons on your keyboard. If you turn those up far enough you'll get distortion as well.