Natures and Characteristics do not have to be of the same stat in order for the Nature to increase one of your Stats by 10%. So for example a Pokémon does not need both a Special Attack Nature and a Special Attack Characteristic in order for your Special Attack Stat to get the 10% boost.
Characteristics do not increase a Pokémon's Stats in anyway, they just indicate which stat contains the Pokémon's highest IV.
As for you wanting to catch a Galvantula with a Modest Nature and an Alert to Sounds Characteristic. Yes that would work since Modest increases Special Attack and the Alert to Sounds Characteristic indicates that your Galvantula's highest IV is in Speed. Electro Ball is a special move and inflicts more damage the faster the user is compared to the target, so this would work nicely.
But remember having an Alert to Sounds Characteristic does not guarantee that you will have 31 IV's in Speed, in fact your IV's in Speed could be either 1, 6, 11, 16, 21, 26 or 31.
Hope I helped :)