How to achieve smooth sound fading effects?

What is the best possible way to smoothly fade in/out a sound with SFML 1.6?

It has SetVolume() and SetAttenuation(). These two directly set these values. But I need a开发者_运维问答 smooth transition from one sound to another.

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I think the best way would be to set the volume lower and lower with each frame/time step.