I think this is a type that has been mentioned and played around with a few times by others, I think it's a good one because it makes sense and fits with so many different pokemon.
Name: "Sound Type"
Weaknesses: "Steel", "Rock", "Ground", "Ice"
Reason: These types are solid enough to dampen sound and prevent sound waves from passing through them.
Strengths: "Fire", "Water", "Normal", and "Fight"
Reason: These types allow sound to pass through them or are physically affected by sound, making them susceptible to sound waves.
Immunities: None
Reason: Everything can create and affect sound.
Existing Pokémon that you'd change to this type:
Normal Types that should lose their Normal type and gain Sound type: Whismur Family, Chatot, Audino, and Meloetta.
The Kriketot family deserves the Sound type in addition to being Bug type.
The Noibat family should lose its Flying type and gain Sound type.
The Voltorb family as well as Plusle and Minun deserve Sound type in addition to being Electric.
The Misdreavus family deserves the Sound type in addition to being Ghost type.
Bellossum specifically deserves Sound type in addition to Grass type as well as Maractus.
The Chimecho family and Meowstic family deserve the Sound type in addition to its Psychic type.
The Bronzong family should lose its Psychic type and gain Sound type.
Sudowoodo specifically deserves the Sound type in addition to being Rock type.
All of the pokemon I have listed above are based on either making a lot of sound, dance frequently, or are mentioned to operate under some form of rhythm.
There are also many different moves that within their descriptions are obvious choices for conversion to being sound type instead of their original type. There are way too many for me to list here.
