Strongly recommend to use the nRF5 SDK drivers to access the peripheral instead of configuring it directly. Using drivers will also help you understand their usage and give you more confidence to configure then directly in future (if you need to)
The driver for I2S for example have init function you have nrfx_i2s_config where you can set the interrupt priority.the second argument will plugin your handler when the interrupt handler is called and When stopped, release all buffers, including these scheduled for the next transfer.
