Years of practice and an innate musical talent has granted you the ability to rend your foes with musical notes like a blade. 

Rhythm of the Fight
Musical instruments are now considered natural weapons for which you are proficient.  Attacks are Thunder damage using charisma as a modifier and has a special reach property.
Attacks within normal melee range use a 1d8 for damage.  For each additional 5ft of range the damage is reduced by 1 die size.  (1d8 > 1d6 > 1d4)

Masterful Musician
Instruments grant their bonus to attack and damage when used as a weapon and if used as a spellcasting focus, spell attack and DC.
You may select any one of these mastery effects when you make an attack.  With expertise you may combine any two. 
Expertise adds your charisma modifier to Thunder damage dealt by spells cast through your instrument.