Use this tool to generate a php random midi player script.
On the next page enter a base url and the relative urls for your midi files.
Then take the generated script and save it as a php document.

To add it to your sig box use the url for the file in an embed.
<embed src="" hidden="true" autostart="true" loop="-1" />

Your base url should look something like this.
http://your_domain/sigs/

The relative urls for your midi files would look something like this
freebird.mid
Or if they are down another directory it would look something like this
classic/freebird.mid