I think a solid-state MP3 player such as an iPod Nano would be the most robust, then. Triggering its play button via a relay connected to its headphone jack would probably be the most reliable thing. And because solid state MP3 players are cheap and plentiful, you could, in theory, have multiple hot spares on hand if one malfunctions.

What you haven't described is the mechanism by which the fireworks launch system can trigger the play function on the audio. Is it a relay wired up to a connector or what? I don't know what these systems look like. You'd think that if the system had a way to trigger audio-start, that they'd also *already* sell an interface just for this purpose.