What has happened to Gregor's voice?

From the text:

The soft voice! Gregor was startled when he heard his voice answering. It was clearly and unmistakably his earlier voice, but in it was intermingled, as if from below, an irrepressibly painful squeaking, which left the words positively distinct only in the first moment and distorted them in the reverberation, so that one didn't know if one had heard correctly.