yeah it was caused by the different refresh rates of pal & ntsc
early games had a noticeable difference in the sound but then they started programming pal games with faster sound, the actual code would be faster than in the ntsc version to compensate so they both sound the same when run on the correct consoles, if you can find pal & ntsc roms of the same game and run them in a nes emulator in ntsc mode you'll notice the difference in the sound, the ntsc version should sound fine and the pal version should be faster (the pal version might also be in a slightly different key as well iirc) run them in pal mode and the pal version will be ok and the ntsc will be slow, if it's an early title then they should both sound fine in ntsc and slower if you run them in pal as the sound was programmed identical