Al = Triangle, Tamborine
Ed = Two-handled guitar! (I dunno what you call it.) And harmonica, strangely enough.
Sloth = Harp (I'm not kidding...this is silly but I'm not kidding)
Trisha = Instrument? I'd have her singing!
Lust = Violin (And I play that too! XP) or Tamborine, like a gypsy.
Gluttony = He'd eat it. But Timpani
Pride = Can you imagine him evilly playing an organ?
Greed = Electric guitar, but shaped weirdly in some evil form.
Wrath = Dora the Explorer drumset. BWAHAHAHAHAH! Okay, kidding. He'd play drums or something.
Envy = Drums, like Wrath. They both have a lot of not-so-repressed rage.
Roy = Acoustic Guitar, but maybe amped up a little. Or maybe a base electric guitar. I can also see him with shakers. Actually, anything Bossanova works with Roy
Riza = Somehow, I can see Riza with a mandolin or acoustic guitar, maybe a lute. (Have you seen the scene in "Breakfast at Tiffany's" with Holly playing "Moon River"? I could totally see Riza doing that.). Or a flute too, for all you know. Something that's got a bit of an earthy yet delicate tone to it. Oh, and maybe a piano too.
Hughes = Ukelele! BWAHAHAHAHA! Or harp...seeing as he's in heaven now. *cries*
Havoc = Same as Ed, Harmonica. Or panpipes.
Falman = Cello. Something very dignified.
Fuery = Recorder!
Breda = Trumpet or Sax. And maybe a Banjo, for some reason.
Armstrong = Tuba, Oboe. Something loud and flamboyant, like he is.
Elysia = Okay, I am so sure that Elysia would have been a xylophone or marimba girl.
Nina = Bells!
Scar = Bone flute. Something lonely-sounding.
Dante = I could totally see her playing the evil organ with Pride, her greatest work.
Hoheinheim = Sang duets with Trisha. Also, something old like the harpsichord or a wood organ. A not-evil organ.