How to get the audio line-in data from the STM32H735G-DK?

I tried the BSP example for H735G-DK, but it doesn't work.

What I want to do is get an audio wave from line_in (the blue audio jack on the board).

The setup for SAI1 is in the picture, I can play the audio through the line out without any problem but can not get the readable waveform data from the line in.

uint8_t txData[2] = { 0, 0 };
/* enable the clock by starting the tx */
if (HAL_SAI_Transmit(&hsai_BlockB1, txData, 2, 1000) != HAL_OK)
 Error_Handler();
 
if (audioDrv->Play(audioCompObj) != 0)
 Error_Handler();
if (HAL_SAI_Receive_DMA(&hsai_BlockA1, (uint8_t*)recordBuff, RECORD_BUFFER_SIZE) != HAL_OK) {
 Error_Handler();
}

The audio input is a 2k sin waveform, what I got from SAI1 are:

It does have the 2k frequency peak, but the waveform is far from a sin wave, what could be the problem?