CrossRoads:
but then you'll cripple performance with
"I will try using a dedicated audio codec using the I2S interface." with its 400Kbit/sec max serial transfer speed.
why?
Because I2S is specifically designed for handling high-speed digital audio, such as between a CD transport and an outboard DAC.
Your estimate of the bandwidth is way off, I2S uses clock rates around 2 to 12MHz (depending on sample rate, word length and number of channels). It can cope with 24/192 so is entirely capable of 12/30.