The audio remains a high point. The soundtrack, composed by the legendary Toshikazu Tanaka, retains its driving, urgent military jazz. The sound effects—the chink of the heavy machine gun, the scream of "ROCKET LAUNCHER!", and the dying gurgles of enemy soldiers—are preserved in their original glory, providing that authentic arcade ambiance.

Just as Morden is captured, a mysterious portal opens in the sky. An army of "Future Soldiers"—Morden's own troops from a future timeline—steps through to rescue their leader.

Critics at the time noted one persistent issue: slight input lag. On original Xbox 360 hardware (and via backward compatibility on modern Xbox consoles), some players feel a 2-3 frame delay between pressing jump or fire and the action on screen. In a game where milliseconds mean life or death against a wall of rockets, this is frustrating. SNK’s emulation of their own hardware wasn’t always perfect, and Metal Slug XX suffers for it.

Faced with this technologically superior threat, the Regular Army's elite Peregrine Falcon (PF) Squad Sparrows Unit