Don't want to write this by hand? GodCraft sets minecraft:instrument_sound for you with a switch.Get GodCraftOverview
minecraft:instrument_sound is a block component: it goes in the components object of a custom block. Picks the sound a note block makes when this block is above or below it. Set "up" and/or "down" to a note sound; use "note.none" for silence.
Example
Inside the components object of BP/blocks/<name>.json (values are the defaults or placeholders):
BP/blocks/<name>.json
{
"minecraft:instrument_sound": {}
}