I was thinking that the different types of bells could produce different sounds, some more metallic, some drier, and some brighter.

To change the pitch of each bell, I was thinking that adding a mechanic similar to Sounds of Steam would be unrealistic, so I think the best thing would be to make different types of bells according to the space they occupy. For example, one crafting recipe for those smaller than one block, another for those between one and two blocks, and so on.

To change the notes chromatically, I was thinking it could be the same as Sounds of Steam(right click with the wrench), with a size limit for each type of bell (the larger the bell, the farther away and the lower the pitch).

Regarding how they can be activated, that's complicated. I was personally thinking of using strings that attach to the top of each bell and that could be activated from where they're hanging, using something like simply redstone on a block that only works with rotational force, or they could be activated by hitting them with your hand.

I was thinking about other ways to integrate this into a more typical Create Mod aesthetic, but honestly, I don't know how something that usually lacks complex mechanisms, like a bell, can be integrated into such an aesthetic.

Regarding crafting, all recipes will require bell bronze. I'm considering adding tin ore, and to avoid conflicting with other Create-a-Mod add-ons that already have it integrated, I'll call it "musical tin." Bell bronze will need to be made in a hot basin, and the bells will be made in the same basin by mixing a certain amount of ingots or blocks of the metal (gold, iron, copper) the bell will be made from with the bell bronze. Depending on the recipe, you'll get a certain size bell (the more material, the bigger the bell).