This is, fairly self-explanatorily, the ability to do music at more than a casual level.
I propose this skill be made available as a background skill, as well as as a course of study in undergraduate/graduate school.
Musicians should take points in disciplines and instruments - for example, a character with Musician 8 could put all eight points in one instrument, four into one and two each into two others, learn one with a 3 and five others at a level of 1, or any combination desired.
Music skills cascade at 2:1 within a "family" of disciplines below. For example, a character with "Guitar 6" would have a "3" on the bass, mandolin, banjo and the other instruments in the family.
The families are:
Voice
Percussion - Drums and percussion instruments (from tympani to triangle), plus xylophones, marimba and chimes.
Keyboards - Pianos, Organs, and other keyed instruments.
Fretted Stringed Instruments - Guitars, Electric Basses, Mandolins, Banjos, Dulcimers, Sitars, Sazzes, Balalaiki, Ouds and so on.
Bowed String Instruments - Violin, Viola, Cello, Upright Bass, Hardanger Violin and others.
Brass Instruments - Trumpet, Trombone, Tuba, Bugle, and the variations on them.
Single-Reed Instruments - Saxophone, Clarinet.
Double-Reed Instruments - Oboe, Bassoon.
Bag and Reed Instruments - Bagpipes. Characters should pick one specialty type (Highland, Uilleann, Northumbrian, Breton, or any of the many other varieties) - skills cascade with other types of bagpipe.
Woodwind Instruments - Flute, Pennywhistle, Recorder, Harmonica, etc.