If you would like to listen to a voice message at increased speed, first click on the play button as usual.
After you have clicked on this button, a small display "1x" appears at the bottom right of the voice message.

This indicates that the voice memo is currently playing at the original speed. By clicking on this display, you can play the message at 1.5 times the speed and by clicking again at 2 times the speed. If you click the ad a third time, the voice message will be played back at the original tempo.