No Sonic cannot be unlocked with a hammer. All of the stages in the challenge screen can from my experience (did 2), but most of the stages are really easy to unlock anyway, so I wouldn't waste my hammers on them.
I unlocked Sonic before the usual way (300 VS. Matches). Not sure exactly how we got him, but it was after playing on Final Destination with 2 humans (red team) and 2 lv9 cpus (blue team). I think it was most likely caused by playing a certain number of matches on final destination.
Yeah, it's Brawl. I assumed that everyone would figure it out, since Sonic is in Brawl and he has to be unlocked. I don't plan to play Single Player on this version, so what stages can't I unlock? I might use a hammer for one of them then, perhaps? What else would I use the hammer for?
Big Blue (Melee) = Brawl as Captain Falcon 10 times
Electroplankton Stage = Complete Event #28
Flat Zone 2 = Unlock Mr. Game & Watch
Great Sea = Unlock Toon Link
Green Greens = Fight as Kirby in VS mode 20 times
Green Hill Zone = Unlock Sonic
Jungle Japes = Play on any Melee stage 10 times in VS Mode
Luigi's Mansion = Complete Classic Mode on Easy
Mario Bros = Beat Challenge #19
Pokémon Stadium = Play on Pokémon Stadium 2 10 times in VS Mode
Port Town = Play as Captain Falcon 10 times in VS Mode
Spear Pillar = Clear stage #25 in Event Mode
I have every character and stage now. I guess the only thing I really need now are more songs, eh? So, any list of what songs that are unlockable? Also, which are the hardest to unlock? Perhaps I'll use the hammers for them.