When you connect a normal line level device to a Phono jack using an analog cable, the resulting sound may be distorted, too loud, or have too much low-end boost.
To avoid this, simply connect AirPort Express to another jack such as CD, Tape 1, Tape 2, Video 2, AUX, DVD, or DAT.
Tip: This applies only when connecting AirPort Express via analog cables, not digital.