TS-8390-49xx 3.5mm Audio
From embeddedTS Manuals
The TS-8390 includes a 3.5mm audio jack which can be used instead of the speaker by turning off the speaker GPIO.
# Use 3.5mm output and disable speaker
echo 0 > /sys/class/leds/en-speaker/brightness
# Use the speaker and disable 3.5mm output
echo 255 > /sys/class/leds/en-speaker/brightness