Baldur's Gate : The Sword Coast Chronicles : NWN2 Persistent World

Does curse-song level from only bard or does the bard-prestige classes add to its benefits as well?

You know it says "requires 13 levels of bard and so much preform". But will prestige classes substitute for this?

Dissonant chord should. And Stormsinger should if you pick bard progression. It won't work if you go bard3/cleric17/stormsinger10 with cleric progression, for instance.