In the original Super Mario Bros. NES game from 1985, certain short-term events, and the sounds associated with them, often seem to cause fluctuations and distortions in whatever background music is playing at the time. For example, when one picks up a coin at the same time as a note in the background music begins, the note often modulates to a different pitch or is absent altogether after the coin pickup sound ends. Jumping on or defeating an enemy also often produces distortions in the music. Why is this? Is there an explanation related to the sound or hardware limitations of the NES console?
(Admittedly this question may be misguided, as my NES broke several years ago and I've only been playing the Wii Virtual Console version of Super Mario Bros. ever since. But I do seem to recall the glitch happening on the original game, and it's not like they couldn't remove these distortions in the modern version if they wanted to.)
Best Answer
What's causing the modulation is a bit difficult to explain. There's a whole mess of different ways for music/sound to fail during playback, but modulation usually happens when a group of notes is played with incorrect settings. This can be due to programing error, or (in this case) hardware error when the system interrupts a sound channel too quickly to late the previous note decay. I highly suggest checking out this video The 8-Bit Guy has in regards with sound/music worked on older systems!
(skipped to the NES explaination)
The biggest takeaway here is that the NES has 5 voice channels to work with, but four of that five where locked to one type of sound.
In the video, they break down Super Mario Bros 2's background song and shows each of the voices used. What's not shown is where sound effects would come into play.
Doing a bit of research, I loaded up Super Mario Bros and turned off all the sound channels except for one.
The thing to notice here is that you can hear the coin sound effect, but it has to interrupt the song being played. (In this case, I found that the coin sound effect was being played on Voice 2 - Square 2.) Notice that the jumping sound effect isn't heard, (it's on a different voice channel), and the rest of the background song is muted as well. It's possible that the modulation you're hearing is a special instance of picking up a coin at the same millisecond of the track restarting.