all the data has to be converted in software to serial for the I2C bus that the MAS chip requires.
MP3's down a 1-bit serial interface? I'm not sure what the Amiga parallel port is capable of, but I would imagine you could pre-process your MP3's into I2C files you can DMA.
For a fixed rate MP3 that could be 192Kbaud, what is the maximum rate of the Amiga parallel port?