This isn't about fading out the end of a song. It's about making sure that a piece of audio is neat and tidy, as pro work should be.
Let's suppose for instance that you are given a perfect recording of speech to top and tail. (That means cutting out any preamble and postamble, leaving only the wanted material.) The recording is super-clean and super-quiet. So all you have to do is cut it at the beginning, and cut it after the last syllable has ceased to sound. Easy-peasy, no fading required.
But suppose that there is some background noise. Perhaps the piece was recorded in a public place for instance. There are two possibilities...
The first possibility is that the background is low enough in level that the speech completely masks it. Since normal speech is full of gaps, you'll hear the background then, but not when the person is actually producing sound.
Topping is usually easy in any situation - just cut as close as you can to the opening syllable.
At the end however, what you need is a very quick fade that 'chases down' the last syllable as it ends, in terms of the speed of the decay and the shape of the delay curve. Some experimentation might be necessary but the aim is to completely remove any trace of background at the end, without losing one iota of the speech. This would also apply if the recording venue was quiet, but there was some electronically-generated noise for whatever reason.
It might be the case however that the background is audible throughout. Clearly this isn't ideal, but in the professional world you often have to deal with imperfect source material.
In this case a quick fade is not the best solution. The listener will have gotten used to the background throughout the duration of the piece, and fading too quickly will simply draw attention to it.
The solution here is to hold the level for a short period - probably less than a second - after the last syllable, then fade out gracefully taking around another second to do so. Adjust the timings to taste.
Sometimes perfection is unachievable. But professionalism in audio always is. Keep the pleasure of the listener in mind above all else and you won't go wrong.
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