Question about STM32F4 DMA
You are confusing DMA priorities with IRQ priorities. The priorities you set with the DMA_InitStructure only deal with setting the priorities inside the DMA controller, i.e. which of two (or more) simultaneous DMA(!) requests gets serviced first. This is completely independent of the interrupts that get triggered at the middle and/or end of a DMA transfer, which get prioritized by the NVIC (along with the other interrupt sources). As you said, you need to set higher (i.e. lower number) IRQ priorities for the DMA interrupts than for the ADC interrupts to make sure that the I2S related DMA interrupts are serviced immediately.