You can spend mana produced by Altar of the Lost to cast any spell with flashback that you cast from a graveyard. You don’t have to be using flashback to cast that spell, however, as long as something else is allowing you to cast that spell.

Altar of the Lost doesn’t allow you to cast spells from any other player’s graveyard, although you can spend the mana it produces on such spells if something else allows you to.