Glad you found the solution to your first problem! As for the mp3 file, as far as the built-in soundfont playback with SCAMP is concerned, I haven’t yet implemented this. It’s definitely on my list of things to implement as I approach version 1.0, since I know it’s useful to many people.
The reason it wasn’t a top priority is that the main role of the soundfont playback is for prototyping, getting a sense of what a piece would sound like as you work on it. For actually creating a recording, you’re usually going to better results using new_midi_part() to route the MIDI to a digital audio workstation or external program.
Also, you can record the soundfont playback with a little effort, by routing sound playback in your computer. See: