Fission, from Rogue Amoeba
macOS application

This audio application for Macintosh computers is a great aid for language study. Instead of listening to an audio track in iTunes, open it in Fission. The audio is depicted as a waveform (a bit like an ECG tracing) which can be stretched or compressed using a slider. A vertical line representing the playhead moves across the waveform as the track is played. One can stretch the waveform enough so that gaps in the conversation (eg, between speakers, or even between sentences or phrases spoken by the one person) become readily apparent.

The visual representation is a great help when it comes to deciding where to pause a recording or select a segment to replay. Wherever you click in the waveform, the playhead immediately jumps to that position and play proceeds therefrom, so it is very easy to replay snippets at will. Much easier than dragging a slider along a timeline in iTunes where you need to keep looking at the time and remembering where you would like to return to. Just open a track in Fission, do a quick stretch, and start playing. Quick and easy!

You can also chop a track up into separate audio clips if you wish, which is particularly handy for language courses which have quite long tracks and chunks of conversation (which you might wish to replay) mixed in with exercise-type material (which you don’t).

Just to be clear, though: this software does not alter the speaker’s rate of delivery: stretching the waveform does not make the speaker speak more slowly. Alteration of playback speed would require compensatory alteration of sound frequency in order to retain normal-sounding speech, which Fission is not designed to do. I understand that other software is available for this purpose, but it is more expensive and more complicated, and I have no personal experience with it.

Hokusai, from Wooji Juice
iOS app

This audio app for iPhones and iPads offers (among other things) waveform depiction of playback similar to Fission (which is not available for iOS).