I'm not thinking about this too hard. First thing I come up with is 'S':

Schoenberg
Schubert
Schulze, Klaus (L, that alone would be enough)
Schutze, Paul
Seefeel
Shape of Despair
Shotgun Messiah
Shriekback
Sibelius (in classical, are we counting composer, as I am, or performer?)
Sigur Ros
Skinny Puppy
Sky Cries Mary
Slayer
Slowdive
Smashing Pumpkins
Snakefarm
Snog
Soilwork
Spinal Tap
Spiritualized
Split EnzStabbing Westward
Steroid Maximus (Foetus side-project)
Strapping Young Lad
Strauss, Richard (meh, who cares about Johann?)
Suede
Sunday All Over The World (Fripp side-project)
Sunny Day Real Estate
Super Furry Animals
Supertramp
Swervedriver
Sylvian, David
Symphony X
Synergy

...and of course, Britney Spears. That would certainly be enough to get me by. But I'd have a hard time living without:

Tori Amos
Anathema
Black Heart Procession
Calexico
Chick Corea
Al Di Meola
Genesis
Jethro Tull
Kraftwerk
Bill Laswell
Pete Namlook
Nine Inch Nails
Mike Oldfield
Pink Floyd
Planet P Project
Return to Forever
Rush
Talk Talk
Tangerine Dream
Vangelis
Yes
...

I'd probably go with B

Jeff Buckley
Brainiac
Brand Nubian
Black Eyes
Black Moon

etc.

N

New Buffalo
New Order
Nick Cave
Nick Cave & the Bad Seeds
Nick Cave & Warren Ellis
Nick Cave, Mick Turner, Blixa Bargeld
Nick Cave [feat. Anita Lane]
Nick Cave [feat. Dirty Three]
Nick Cave [feat. Lydia Lunch]
Nick Cave; Die Haut
Nick Drake
Nina Simone