Medallion will reduce the cost of flashed back cards.
The relevant rules:
502.22a Flashback appears on some instants and sorceries. It represents two static abilities: one functions while the card is in a player’s graveyard and the other functions while the card is on the stack. “Flashback [cost]” means “You may play this card from your graveyard by paying [cost] rather than paying its mana cost” and “If the flashback cost was paid, remove this card from the game instead of putting it anywhere else any time it would leave the stack.” Playing a spell using its flashback ability follows the rules for paying alternative costs in rules 409.1b and 409.1f–h.
409.1f The player determines the total cost of the spell or ability. Usually this is just the mana cost (for spells) or activation cost (for abilities). Some cards list additional or alternative costs in their text, and some effects may increase or reduce the cost to pay. Costs may include paying mana, tapping permanents, sacrificing permanents, discarding cards, and so on. The total cost is the mana cost, activation cost, or alternative cost, plus all cost increases and minus all cost reductions. Once the total cost is determined, it becomes “locked in.” If effects would change the total cost after this time, they have no effect.
It will not, however, affect suspend cards, because suspending a card is not casting it.
502.59a Suspend is a keyword that represents three abilities. The first is a static ability that functions while the card with suspend is in a player's hand. The second and third are triggered abilities that function in the removed-from-the-game zone. "Suspend N--[cost]" means "If you could play this card from your hand, you may pay [cost] and remove it from the game with N time counters on it. This action doesn't use the stack," and "At the beginning of your upkeep, if this card is suspended, remove a time counter from it," and "When the last time counter is removed from this card, if it's removed from the game, play it without paying its mana cost if able. If you can't, it remains removed from the game. If you play it this way and it's a creature, it gains haste until you lose control of it."
