Sheldon: Jerry Lewis (in his Nutty Professor voice)

Gramp: Ed Asner.

Arthur: Samuel Jackson (just to have him comment about snakes on a plane)

Dante: Sean Penn (doing Spicoli)

Flaco: James Earl Jones.

Sheldon: the 'guy' who does the voice of Italy in Axis Powers Hetalia.

Gramp: Judd Hirsch

Arthur: Sonny Strait (see his role on One Piece)

Dante: Christian Potenza (see his role on 6teen)

Flaco: Bella Hudson (see her role on One Piece)

Sheldon: Matthew Broderick
Gramp: Walter Matthau
Arthur: Closer to Bugs Bunny
Dante: John Di Maggio (Bender) Flaco: The Snoopy voice

Grandpa: Wilford Brimley or Peter Reneday
Sheldon: Wil Wheaton
Dante: James Arnold Taylor
Arthur: Rob Paulson or Cam Clarke
Flaco and Oso: Frank Welker
Other Choice for Flaco: Pat Fraley

Ok... I'm going to go all british on this... purely to add a little sophistication to the thread.

Simon Pegg as Sheldon
John Rhys Davies as Grandpa
Rik Mayal as Arthur

AND

Andy Serkis as Flako

Only 2 came to me right away -
Gramp: Wilford Brimley (even looks like him!) Sheldon: Frankie Muniz

Hubby says Arthur has the voice of Billy Mays. Gramp is Ron Glass, he claims. David also says his favorite Doctor Who (Christopher Eccleston) for Sheldon. David also claims Samuel L. Jackson for Flaco.

I don't see any of that...but hubby's not on the forums and I am.

Flaco has a high-pitched voice in my head with a bit of humor in it. Andy Devine's voice comes to mind for me (an old-time actor, I love old movies).

Spanky McFarlane is what I hear when I read Sheldon - that childish, intelligent voice from Our Gang. With Alfalfa being Dante (I love Dante).

Gramps is a Barry Fitzgerald...hands down.

Arthur is Richard Haydn (he played Max in Sound of Music, but had a nasally, high-pitched voice that was psuedo-intelligent).

Sheldon-Jonatha Taylor Thomas
Gramps-John Ratzenberger
Arthur-Zach Braff
Flaco-Pamela Segall Adlon (voice of Bobby Hill)
Oso-David Tennant
Dante-Johnny Yong Bosch

I've gotta admit, I go through phases. I don't always hear the voices the same way. (Wow, that sounded like I need medical attention)

But Currently, They sound like this to me.

Gramp - William Shatner (Don't judge me!)

Sheldon - Jim Henson (I don't know why, but 'Kermit' seems to sum up his role in the cast nicely)

Arthur - David Tennant (The currently outgoing Doctor's energy and slightly offbeat personality suits Arthur to a 'T'.)

Flaco - Rowan Atkinson (In his 'Blackadder' tones)

Dante - Jim Carrey.

Gramps: Either Wilford Brimley, Paul Teutul Sr. (both have great facial hair) or the old guys from the Muppet Show.

Sheldon: Bill Mummy. He has the geek chops from Lost in Space and Babylon 5.

Dante: Either Kevin Bacon or Christian Slater. Interchangeable.

Arthur: I was tempted to say Daffy Duck but that's too simplistic. John Ratzenberger. Nobody else is as big a know-it-all.

Oso: Lassie, with a bad cold.

Flaco: There are very few people debonair and cool enough at the same time to play Flaco, and also have his self-confidence (remember when he casually "thwip-slapped" a Russian brown bear?). I submit magician David Copperfield, MacGyver (Richard Dean Anderson), Lars Ulrich from Metallica, R2D2 (who could get the most expression out of a simple sound), or, my personal favorite, Tom Baker, the best Dr Who ever.

Dante-Wallace Shawn (Vizzini on Princess Bride)

Gramps - Jack Albertson (Grandpa on original Willy Wonka)

Arthur - Daniel Radcliffe (keep British accent just to make it more interesting and confusing)

Arthur - Neil Patric Harris

Flaco- Toss up between Lenard Nimoy or Scatman Crothers

Seriously, we went 24 hours without anybody naming Peter Cullen, the voice of Optimus Prime himself?

He could do Gramps in his sleep.

Or, just for fun, he could be Flaco. :D

I haven't really thought about the rest of the cast, but swear to god, every time Gramp has a line I hear Doug Price doing Dr. Floyd's voice from The Radio Adventures of Dr. Floyd.

I haven't really internalized most of the voices yet (if I have to, I can hear Gramp's dialogue as read by Ed Asner, but I think that's just because of the recent trailers for Up), but it's patently obvious to me that Flaco sounds exactly like Beaker from the Muppets.

Hmmm. I've never really thought about this because I only make my own voice in my head, but let's see if I can get anywhere. It's hard though because I'm Dutch... so I don't know many voice actors.

Sheldon: Definitely a Matt Damon.
Gramp: He'll kill me, but I'm gonna say Travis Willingham.
Arthur: Vic Mignogna. Because Vic's an Arthur, and Arthur's a Vic.
Dante: Charlie Sheen. I'm not kidding.
Flaco: Whoever does GIR from Invader Zim. None other!

Would we need to cast Oso? Because I want to audition one of my bunnies for him.

PS. I'm really bad at this...

I gotta say, ever since this has been brought up, I've been giving this a lot of thought. Here's what I came up with:

Gramp - Ok, stay with me here - his voice is somewhere between Alec Baldwin and Peter Boyle, with some of my dad thrown in. I know what you're thinking; 1. how do you get from Alec Baldwin to Peter Boyle, and B. who the heck is your dad? Like I said, just go with me on it.

Arthur - At first I was thinking Matthew Perry, but I've seen a few people post Neil Patrick Harris, which, now that I think of it is a much better choice, with MP in a close 2nd.

Flaco - Toughy. It would have to be someone who can get the pitch right, not low, but not too, too high, either. Someone who can express a range of emotion only using the word "squee." I'm thinking Jack Lemmon circa "Some Like it Hot."

Sheldon and Dante - Not really sure what to do here. I just kind of hear my own inner voice here.

!Bonus Character!- The Personification of Wikipedia - Whenever I read this "episode" I always hear Dan Castellaneta as his pimple-faced teenager as a grown-up.

This is pretty close to what I her when I'm reading

Sheldon - Rainn Wilson

Gramp - Samuel L. Jackson. Seriously, I've no idea why, that's jsut the voice that's always come through for my when I read Gramps.

Arthur - Jason Mewes (though a much-cleaned up version)

Dante - Kevin Smith

Flaco - Dee Bradley Baker (voices Perry the Platypus in Phineas & Ferb). Before I started watching this show with my kids, the Flaco voice was one of those little party noisemakers... you know the kind I mean, kinda small, has a little buzzing/zipping sound

Another possibility for Flaco, if we don't have to stick to strictly living people.

Bruce Lee. He got more out of that cat-like "Hoooaaaaw!" than anyone out of a single sound.

For Gramp: Ed Asner.
Sheldon: Frankie Muniz.
Dante: Phillip Van Dyke, one of the voice actors who voiced "Arnold" on the old NickToon "Hey Arnold!" --you know, "hey, football head!"
Arthur: Nicholas Brendon, Xander of Buffy fame.
Flaco: Julie Kavner (Marge of the Simpsons; Brenda Morgenstern)

Oso: The little dog from Frasier

Ooo…
I wanna swap Bobcat Goldtwait for Colin Mochrie in the role of Arthur. Ayersrock2000's right. It's Colin's role. 8^)

Bobby McFerrin as Flaco.

The man of many, many voices.

Sheldon: Neil Patrick Harris
Gramp: Wilford Brimley
Arthur: Gilbert Godfried(sp?) Dante: Henry Rollins
Flaco: Sylvester Stallone. *Squee-Yo!*

When reading comic strips, as an actor/performer/entertainer I hear the characters in different registers of my own voice (since I'm "reading" the strip).

For comic strips adapted to animated cartoons, I either agree with the voices chosen by the producers (Garfield and Dilbert are the voices I hear when reading the strips now) or heartily disagree (Boondocks).

If I were to choose a voice for Flaco though, I would choose either Tom Kenny (SpongeBob, Ratbert, etc) or Jim Cummings (Winnie the Pooh, Tazmanian Devil, etc). :-D