Is it feasible with a Arduino Mega to record 1 channel of audio at 4kHz, or possibly 8kHz straight to SD card?
you'd only have 16000 bytes/second. Too much for 9600 bps, but for 115200 should be fine
And it looks like those calculations were a tad off before:115200 bits per second = 14400 bytes per second
Just another thought. If I use an i2c ADC would that allow me to bypass the SPI baud rate limitation?