That unmistakable boop-boop-boop bass line kicking in. You already know. This is Super Mario Bros. on NES, preserved in the Game Sound Museum Famicom Edition from 2004. "Above Ground BGM" is maybe the most recognized melody in all of gaming, and hearing it isolated like this reminds you how tight Koji Kondo's composition actually is with just a few channels.
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"Underwater BGM" has that waltz feel that somehow makes everything dreamy. And the "Koopa Stage BGM" still gives you that low menacing tension even through tiny NES speakers. The collection includes the hurry-up variants of every stage theme, which is a cool touch.
You can hear how the tempo shift alone completely changes the mood with the same notes. If you need a reason to revisit the most important 8-bit soundtrack ever written, just press play.