Ok, I've checked Ожегов
Let me get this straight.

The genitive plural of "тётя" is "тётей".

As for "песня" there are two words: "песня" and "песнь" (old-fashioned and lofty). Don't confuse them.

The genitive plural of "песня" is "песен".

The genitive plural of "песнь" is "песней".