The 3.5mm audio jack is for headphones or earphones only, it is not able to drive a typical speaker impedance
without overloading, only down to a minimum of 16 ohms. The output power from the headphone channels is
62mW only, inadequate for a speaker.
Note the headphone jack "ground" is not ground, and should never be connected to ground as it has a DC offset,
meaning that trying to use the jack as line-outs could lead to damage in some situations.
The line outs are the outputs for further signal handling, such as connecting to an audio amplifier for driving
speakers. The audio shield has DC-blocking capacitors on these lines so they are ac only.
"Line level" signals are what are used in audio for moving analog signals around between equipment such as
preamps and amps. The SGTL5000 maxes out at about 1Vrms on all outputs, note. Line level signals normally
expect to see load impedances of 10k or higher.