Sing, sang, sung, song is indeed an example of ablaut, but only one kind of ablaut. There is another more interesting kind because it's more fruitful. This is shown by the words nest, sit, seat.
The root here is 's-t'. The nest is where a bird sits down. ni means down, the same as in the words nether and beneath. This type of vowel gradation was very productive, and is still present in North Indian languages, as in beithna, to sit, bithana, to seat.
I can't find a reference, but I suspect the word ablaut was introduced by the brothers Grimm, serious philologists in addition to being renowned for their fairy tales.