What do you mean by "spellsong"? Are you working with a bard PrC or something? The bard's Perform-based abilities (like Countersong, Inspire Courage, etc) are not actual spells and can't be put into magic items with the standard rules.There is a feat that can increase a spellsong's duration by a certain amount when the spellsong is cast. The amount that it is extended by depends on a Perform check.
Putting that aside, do you have a description of the feat you're working with? It doesn't sound like something I've heard of before. It's entirely possible that you've just come across a badly-designed feat. In general, only [Metamagic] feats modify spells at all. (The exceptions are things like Spell Focus, which modify all (or a large subset) of the character's spells at once.)
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