AV Jack can be an area of confusion so we are mapping the pinout.
1 being the tip and 4 being the sleeve.
CTIA Headset Standard (US Headjack Standard)
1 Left
2 Right
3 Ground
4 Mic or Composite
OMTP Headset Standard
1 Left
2 Right
3 Mic
4 Ground
From the hardware schematics: Libre Computer Board Hardware Schematics Links
ALL-H3-CC
1 Right
2 Left
3 Ground
4 Composite
AML-S905X-CC
1 Left
2 Right
3 Ground
4 Composite
ROC-RK3328-CC
1 Right
2 Left
3 Ground
4 Composite
Raspberry Pi 3B/3B+/4 (some official sources conflict each other 
)
1 PWM1
2 PWM0
3 Ground
4 Sleeve