As you have it configured, it's set up for a 12 dB gain (GAIN_SLOT to GND).
If you are sure about the I2S signals you are generating, I would check two things:
- That you have set the "SD MODE" GPIO to 1. If it's 0, the amplifier will be in shutdown.
- That the value of R5 is correct. Check the datasheet. What is the logic high level of your GPIOs? 370k would be the "small" value for 5V logic, selecting the right audio channel. If your logic is 3.3V, it doesn't correspond to the recommended values and so I wouldn't be sure of the behavior in this case.