sonic is already included in espeak-ng, but can also be used standalone,
for instance to speed up or slow done reading an audio book.

For instance, to multipy by 2 the eading speed of an audiobook then
listen to it type:
sonic (s2 audiobook.wav audiobook_faster.wav
play audiobook_faster.wav

To know more type: man sonic

Examples are provided in the samples sub-directory.
