How to stretch high frequency digital signals (e.g., MIDI) into the audio range
Since MIDI data appears to be a series of 8-bit data bytes as sent by a standard UART, you could use a microcontroller to receive the data at the standard MIDI rate, and re-transmit it at a lower baud rate.  This assumes that there are sufficient gaps in the data that you won't over-run the receive buffer.  

This would probably require a microcontroller with two UARTs, as the UARTs I've used don't allow different transmit and receive speeds.