In 2013, a post on Supper Mario Broth showcased the original voice of Yoshi’s voice actor, Kazumi Totaka, restored by slowing down the voice samples used in Yoshi’s Story by 50%. However, that post did not include every single sample Totaka recorded for Yoshi, as not all of them have been extracted from the game at the time. Thanks to a new rip by sounds-resource.com user “Cooper B. Chance”, we can now hear all of Totaka’s samples in original quality.

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Super Mario Maker makes extensive use of multi-track songs in its soundtrack. The Edit themes all consist of 8 tracks playing simultaneously and fading in and out randomly to create a dynamic mix. However, it is not only the Edit themes that use this technique. Every single track playing in the Course World section of the game (Courses, Makers, and all 100 Mario Challenge themes) is actually a separate track of the same song fading in and out as the player changes between the subsections.

Here is the song with all tracks playing simultaneously, which is impossible to hear in-game. Despite being designed to be played separately, the tracks are harmonized with each other.
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In Mario Kart 8/Mario Kart 8 Deluxe, Yoshis can be found on the sides of several tracks. Randomly, some of the Yoshis will hum a melody that is barely audible when the player drives past them. The melody is actually Totaka’s Song, a short tune present in nearly all soundtracks Kazumi Totaka, Yoshi’s voice actor and longtime Nintendo composer, worked on. The Yoshis hum the melody three times in slightly different ways before looping. This sound file was extracted from the game’s files and consists of two full loops.
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The track “Super Brothers” from the 2000 Sega Dreamcast game Jet Set Radio (also known as Jet Grind Radio in some territories) contains oblique references to the Super Mario franchise. According to the game’s composer, Hideki Naganuma, the lyrics are as following:

“Come go the rescue Peach
The super boy
1, 2, 3, 4, 5, 6, 7
He like mushroom
He’s a playin’ in another dimension
Love love love baby
Who are you? What’s your name? Super Brother”
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In the intro to Super Mario Galaxy, Princess Peach reads a letter she sent to Mario out loud, where she mentions that she would be waiting for him at the castle on the night of the Star Festival. In the ending to New Super Mario Bros. Wii, this is referenced by having her read the same letter, but become interrupted by Bowser halfway through. Notable is the fact that the voice sample is not reused; listening closely, we can hear that the line is pronounced differently in the New Super Mario Bros. Wii version, meaning it was re-recorded for that purpose.
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On October 28th, 2017, the official Nintendo LINE account published an audio file titled “Jump Up, Super Star! Short Version Free Download”. The audio was a 47-second long sample of the background track of the “Jump Up, Super Star!” song from Super Mario Odyssey, with pitch-shifted Toad sound effects overlaid on top of it to resemble singing. The account then revealed that the song was supposed to be sung by Kinopio-kun, the green Toad mascot of the account, and posted a picture of Kinopio-kun dressed as Pauline (seen as the album art on this post). Note that the Nintendo LINE account is operated directly by Nintendo of Japan.
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Super Mario Odyssey contains an unused speech synthesis function for some of the friendly species in the game. Bubblainians, Steam Gardeners, Volbonans and Tostarenans each have a list of 81 syllables - the ones most common in Japanese - that they can pronounce independently and string into words. In the finished game, this is not used - the species in question merely say random syllables from the list of available ones.

In this recording, I use the files to synthesize the characters saying “Supa Mario” (as “Super Mario” is pronounced in Japanese). In order, the voices are: male Bubblainian, female Bubblainian, Steam Gardener, large Volbonan, small Volbonan, and Tostarenan. You may notice the Tostarenan sounding completely unlike the ones in the game; this is due to the speech synthesis feature for that species being entirely unused and replaced with completely different, high-pitched sound files in the release version.

_{Main Blog | Twitter | Patreon | Store | Source: sounds-resource.com user “Ziella”, second section}

Audio recording of a 1996 preview video tape released by Square for Super Mario RPG. Of interest are the unique arrangements of the tracks playing in the background.

At 0:02: Intro theme (In the Flower Garden)
At 0:22: Mushroom Kingdom theme (Hello, Happy Kingdom)
At 1:16: Normal battle theme (Fight Against Monsters)
At 2:54: Tadpole Pond theme (Grandpa and the Delightful Tadpoles)

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The short operatic track that plays when using the Battery Thing Card in Paper Mario: Color Splash. Many interpretations of the German lyrics exist on the Internet, however, I believe that as a native speaker of German, I am at least somewhat qualified to give my own. I believe the lyrics are “Höret, ihr Narren, das aufgeladene Wort”, which translates to “Hear, you fools, the charged-up word”. “Charged-up” in this context refers to electrical charge, the source word being the same word one would use for a fully-charged battery.

In Wario Land 4, the music in the Domino Row level is deliberately slow, and speeds up to a tempo more common for level music only when Wario activates the dominoes. However, due to the dominoes being a timed event, it is impossible to listen to the full song at this speed. Here is an entire loop of the Domino Row music at “active dominoes” speed. (Sound recorded by me in a GBA emulator.)