A Sound/Music Type is something people have speculated about for years, and it's a move property already in the games. Some people have thought Sound might've been considered as a type back in Gen 3 thanks to the Whismur line, but aside from that there's not much else to support a Gen 3 Sound type until we get more leaks relating to its Source Code and development. However, looking into some things, and a recent observation made by that guy who reviews Pokemon Bogleech, I think there's a decent possibility that Sound was originally going to be a third new type to have come with Generation 2, either in the place of Dark or Steel, or alongside the two.

The main reason for this belief is the large amount of scrapped/early with an odd Sound or Music theme to them. We have a Kokopelli mon, a fertility god who plays a flute, a literal musical note, a kookaburra with a megaphone, and a Xylophone sea monster. These are four Pokemon with explicit sound/music themes to them. If it was just a pure coincidence it's certainly an odd one. (And before you think that 4 mons is too low to justify a new type, remember when Steel was added, there were only 5 FE mons for it, and one was Magneton. And less Ghost and Dragon mons.)

The blue circled mons are just other mons in this batch that POSSIBLY could've been this music/sound type, though I wouldn't say definitely so and they aren't the core mons that I believe support this theory. They're just interesting designs that might clue us in, Remoraid for example might've drawn from a Pistol Shrimp, the loudest animal in the world, Politoed's early design might've had this in mind to replace its fighting type (Politoed's dex entries to this day still talk about its singing), a hula dancer, the Mothman's wing/ears/limbs might've influenced sound, the weird plant mon could've been seen as a microphone, and the two cats have bells. With the more ambiguous mons that never even made it to Spaceworld it's hard to tell what was gone for here.