How do I make it so a keyboard button does a thing but ONLY if you sit in a certain seat?

Well, simply use 'occupant' property of seats.

local UserInputService = game:GetService("UserInputService")
local Player = game.Players.LocalPlayer --> Local script!!

local Sound = game.SoundService.Sound

local Seat = workspace:WaitForChild('Seat') -- Your seat
Active = false

function onInputBegan(input,gameProcessed)
    if input.KeyCode == Enum.KeyCode.C then
        if seat.Occupant and Seat.Occupant.Parent.Name == Player.Name then
            Active = true
        end
    end
end

function onInputEnded(input,gameProcessed)
    if input.KeyCode == Enum.KeyCode.C then
        if seat.Occupant and Seat.Occupant.Parent.Name == Player.Name then
            Active = false
        end
    end
end

Seat.Changed:Connect(function(Prop)
    if Prop == 'Occupant' then
        local Occupant = Seat.Occupant
        if Occupant then
            -- Nothing
        else
            Active = false --> Player left the seat
        end
    end
end)


UserInputService.InputBegan:connect(onInputBegan)
UserInputService.InputEnded:connect(onInputEnded)

while wait() do
    if Active == true then
        -- Code
        if Sound.IsPlaying == false then
            Sound:Play()
        end
    else
        if Sound.IsPlaying == true then
            Sound:Stop()
        end
    end
end