To get Left and right channels stereo, solder short thin AWG30 "wire-wrap" wires to the pins 4 and 6 of the IC. These can go directly to the inputs of a stereo amplifier module board like the PAM8604. Connect the ground pins of the stereo and the GPD board. This GPD board can run on +3.3V OK, as can the PAM8604 audio amp. The module's default mono 2-watt amp plays Left channel only. Jump between pin 6 and C7 (input to mono 2-watt amp) using a 10K resistor in order to have both Left and Right channels in mono.
A 100K resistor can be connected through a push-switch to the 22K resistor (223) to cycle through four different EQ settings. The default setting is about the best: the rest are bass boost: mid-boost: and treble boost.
This device will enter the subdirectories of the SD card and play the MP3 files found there. Press S1 multiple times quickly to go back 15-20 songs. Press and hold S3 to raise the volume: S1 to lower the volume.