The Voices of "Sheldon"

Sheldon: Matt Damon
Gramp: The late, great Walter Matthau
Arthur: Hank Azaria (I'd love to see what he'd do with it!)

Dante: Ben Affleck
Flaco: James Earl Jones. Obviously.

Sheldon: Billy West (He did the voice of "Doug") Gramp: John Mahoney
Arthur: Micheal J. Fox
Dante: Keanu Reeves
Flaco pick 1: Morgan Freeman
Flaco pick 2: Steven Wright

I've always heard Patrick Warburton (Puddy from Seinfeld, the live-action version of The Tick) as Flaco.

My line up would be:

Sheldon:Zachary Tyler Eisen
Dante: Michael Cera
Gramp:Bill Murray
Arthur: Billy West
Flaco/Oso: Frank Welker

See, the voices I "hear" are just sort of generic stereotypes, not any particular person's voices, although I suppose if I heard the right voice I'd recognize it.

Gramps has kind of a medium-low pitched, somewhat gravelly voice, but only a touch, not too extreme. He may be getting up there, but I don't think he's gone, yet!
Sheldon I kind of figure on being probably slightly lower than a 10-year-old's voice normally would, with a bit of nasal quality and maybe some pitch-lightening to it.
Arthur I figure to be a bit more brassy and harsh, although again, not too much so.
As for Flaco, I'd just go with one of those squeezable rubber ducky toys and the sound they make. Squeee...
Dante I just imagine as a bit more left of center and slightly more accented to distinguish his voice from Sheldon's.

Here's what I hear when I read Sheldon:

Sheldon: Drew Carey
Gramp: the dad from Fraiser
Aurthur: Gilbert Gottfried (he has lots of bird voice experience, Iago from Alladin and the Aflac duck) Dante: Will Arnett
Flaco: Nancy Cartwright (Bart Simpson of course)

The voices I hear aren't really voices per se. I mean, they are, but it'd be almost impossible for me to find a match in the real world. They're really fine-tuned. It's hard to explain, but I can't find any actors that'd cover any of these characters. The only character that's gone from paper to screen that'd I've been 100% happy with the voice acting is Garfield. The 80s Lorenzo Music Garfield. The guy nailed it!

Sheldon- Haley Joel Osment
Gramps- R. Lee Ermey (idk why just seems like a good idea)

Dante- Jonah Hill
Arthur- Matthew Broderick
Flaco- I'm torn on Flaco, my choices are both sadly passed but i pick Don LaFontaine or Harry Carey (old chicago cubs radio announcer)

Dante could be interchangably Corey Feldman with a surfer/Zen accent or Boober from Fraggle Rock.

I feel like Flaco's "squee"s would best be portrayed by a particularly ripe chew toy in the shape of a fish or NY Puppy Times. But when he turns a corner, or is engaging fellow lizard, it translates to more of a Charlton Heston or Patrick Stewart.

When Sheldon was first introduced in 'Four Food Groups,' I always saw him as a male Marcie, a straight and more subdued comedic fold to his buddies. Since he sold Sheldonsoft, however, I imagine a less hyperactive and friendlier Dexter (ala Dexter's Laboratory).

Gramp would be a crotchety Alan Arkin or Bob Hoskins. Although I have to give major props to the Walter Matthau suggestion. Oooh, or John Ratzenberger...

For Arther, I think Alan Ruck (a little more from the throat) or Chip Zien could handle the job.

Sheldon: Mark Hamill - got that child-like thing going on.
Arthur: Bruce Campbell - rich voice with a little bit of wiseass.
Flaco: Joe Pasquale - British squeaky voiced comedian - look him up on Youtube, it's bizarre.
Dante: Scott Grimes - if you've heard him on American Dad, you know why.
Gramp: Ernest Borgnine - the quintessential "Gramp".

Gramps: Paul Newman
Arthur: Tommy Chong
Flaco: Rosie Perez
Sheldon: Scott Kurtz (using his Stanley voice)

Gramp: Richard Riehle
Arthur: I still love Peter MacNicol for this voice
Dante: Angus T. Jones
Sheldon: Jim Parsons (with his voice digitally pitched higher)

Flaco: Yeardley Smith is the only person I can think of who might get in the right range.

Sheldon - Fred Savage (The Wonder Years version)

Dante - Christian Slater (The Legend of Billie Jean version)

Gramps - William Demarest (Uncle Charlie from My Three Sons)

Arthur - Tom Hanks (Big version)

Flaco - Rob Paulson (Pinky from Animaniacs/Pinky and the Brain)

Alternative thoughts
Arthur - his namesake, Arthur Fonzarelli
Gramps - Nicholas Colasanto (Coach from Cheers)

Flaco - Leonard Nimoy

Not sure about the rest off-hand, but Nimoy is the only one who could add the gravitas needed to Flaco's more sardonic "squees". Who's with me?

Gramp: Ed Asner
Sheldon: Frankie Muniz
Arthur: Bobcat Goldtwait
Dante: Jason Earles
Falco: Lee Evans

Grandpa-Edward Asner
Arthur-Fran Kranz
Sheldon-Sean Marquette
Dante-Johnny Galecki
Flaco-Frank Welker

Mike Tyson as the voice of Flaco.

'Nuff said.

Gramp: Wilford Brimley. I would have said David Joseph Rufkahr, who played Frank Bartles in the old commercials, but he's gone to that great front porch in the sky.

Sheldon: Charlie Brown from the Charlie Brown Christmas Special. NOT an annoying, cute, child actor--I think of the voice of Sheldon as a legit kid. If you cast one of those Home Improvement types, I will have to shave "Dave Sucks" into your pug's back.

Dante: Edward Furlong (think of his "moi" voice during Terminator II). Dante has a stoner kind of vibe. I would also consider Brian O'Halloran from Clerks, whose voice might sound more legitimately kid-ish than Furlong's now, after Furlong's years of prolonged experimentation.

Arthur: Colin Mochrie. C'mon, this is Mochrie's secret identity, I swear. Have you ever seen Colin and Arthur in the same room?

Oso: George Clooney (note to his agent--yes, he did a dog voice in South Park, but he won't be typecast, I swear).

Flaco: Debra Winger...Pat Welsh for the squees.

Dante: Oops, wrong Clerks actor--
I meant Jeff Anderson / Randal, NOT the guy who actually played the Dante in Clerks!

I'm going to have to agree with Walter Matthau for Gramp . . . and I haven't thought much for the rest, but I occasionally read Arthur with the voice of Patrick Stuart.

Sheldon - Jonah Hill
Dante - Michael Cera
Gramp - Adam Carolla
Arthur - Neil Patrick Harris
Flaco - Nathan Lane

Sheldon - Nancy Cartwright
Dante - Ewan McGregor
Gramp - James Earl Jones
Arthur - Neil Patrick Harris
Flaco - Meryl Streep

Sheldon: Mike Krahulik
Gramp: Mike the Cabby
Dante: Zach Galifinakis
Arthur: Patton Oswalt
Flacco: Daniel Day Lewis

Gramp: as it looks like some others have said: John Mahoney
Arthur: Mark Hamill
Dante: Neil Patrick Harris
Flaco: Ben Burtt (that is to say, Burtt would engineer a really awesome soundset for Flaco's squees like he did for WALL-E).
Sheldon: Gosh, I don't know.
I'll just say Nathan Fillion, because he could always use the work :P

Gramps: Judd Hirsch

Sheldon: Michael J. Fox in "Back To The Future". Alternately, any normal not-too-snarky child actor. Sheldon needs to be played straight, not as a wisecracking smartmouth like you find on most PBS kids shows.

Arthur: Adam Corolla

Dante: Keanu Reeves.

Flaco: Gosh, he's got so many squeees that it's impossible to choose just one person. I'll go with Peter MacNicol as a regular, with frequent guest appearances by Kenneth Branagh, Arnold Schwarzennager, and Jackie Chan.