Using the Music Shield with another microcontroller

If the Music Shield is connected to an Arduino, can you control it sending signals to one of the open pins from a second Arduino?

I have an Arduino Robot I would like to add sounds to. My Music Shield is connected to a second Arduino. Can I send control signals from the Robot Arduino to the Music Shield Arduino?

What signals would I send to select and play tracks on the sdcard?

The shield uses almost all of the pins, but there are D0, D1, A4, & A5 open. Can I use these pins to input commands from a second arduino?

Is there a way to programmatically select specific MP3 files?

For example, when my Robot bumps into a wall, its bump sensors alert the arduino which controls the motors. Can that arduino then send a signal to the Music Shield Arduino to play a particular sound effect MP3?

thanks for your help,

m0j0