Zapp is an extension built to work with WhatsApp Web. It adds controls to the top right corner of your screen which let you change the speed and the volume of recordings.
Start by pressing the icon on the right of the address bar after you entered WhatsApp Web. A box will appear wirh all the controls needed to tweak the audio recordings in WhatsApp Web.
To change speed or volume, just press on the arrows, or scroll the mouse wheel.