Maybe these have been out for a while, but they're new to me:
Some prototypes for Contra: Shattered Soldier and NEO Contra recently appeared on my radar, and to my surprise one of the Shattered Soldier discs (the "2002-05-08" version) has some early music drafts on it. The few differences are mostly minor, where it's not quite the mix heard in the final game or maybe there's an extra few seconds of material on some tracks.
There also seems to be one entirely new track that I haven't previously heard, but aside from that, the biggest surprise on the prototype is an extended version of "LANCE LULLABY." What you hear in the final game and on the OST is a <1-min loop of an incredibly phat synthgrunge beat that plays over the cutscene where you discover the former original Contra P2 Lance Bean is the commander of the Blood Falcon enemy organization.
In the sequel, NEO Contra, you hear what I originally thought was a new, remixed version of this track when (again) the true enemy commander is revealed in a cutscene (though it's obscured by a delightfully arch Steve Blum performance).
But it turns out it's actually the original track from the Shattered Soldier prototype! Wow!
Another interesting bit is the phrase you hear at 1:36 in the embed of the prototype track gets reused/referenced in the NEO Contra track "Nu Metal"

