I was bored so did some Googling. The pins defined in the only demo code that I could find are:
#define AUDIO_I2S_PORT I2S_NUM_0
#define AUDIO_I2S_MCK_IO -1 // MCK
#define AUDIO_I2S_BCK_IO 42 // BCK
#define AUDIO_I2S_LRCK_IO 2 // LCK
#define AUDIO_I2S_DO_IO 41 // DIN
Which seems to agree with the comments in this thread: