"It's Showtime! (Alternate Mix) - Undertale" is a high quality rip of "It's Showtime!" from the game Undertale.
In this rip, the melody of the advertised track is replaced with the melody of Koopa Beach theme from the game Super Mario Kart. The joke is here that both these songs are in the key of C major and follow a very similar chord progression. This rip is also a "melody swap" rip because in the rip "SNES Koopa Beach 2 - Mario Kart DS", the melody of its advertised track is replaced with the melody of "It's Showtime!".
At 0:38, the melody is replaced with part of the melody of "Meet the Flintstones" before freezing up and then the rip is cut short. This is because this rip was uploaded during the Reboot, when rips featuring "Meet the Flintstones" couldn't be uploaded to the channel.