I was going to say what Xav said, but he beat me to it.
If you look at Abe though, he is always talking. Either when rescuing the Mudokons or when he is reciting the FMVs. Of course we don't hear much of the other Mudokons other than Alf at the end of Abe's Exoddus, but I always get an impression that they are more quiet than Abe is [whether this be because of their slavery conditions or for other reasons]. Even when Abe was in slavery he seems more "quirky" than the others. Like at the start of Abe's Exoddus when he is cleaning and listens in on the Glukkon's conversation about Mudokon Pops, there is just an impression of him being more light-hearted [the way he sneaks around, etc]. So tying that into how OWI said that it was done as a child, he probably was a child who made more noise than the rest.
Another possibility is that it was used in a testing situation, to see if such a restraining measure would be any use. As we know, Abe speaks through his stitched mouth, so the restraint wouldn't work. Therefore no more Mudokon children would have had their mouths stitched as it was ineffective.
Abe Babe...