WhatsApp is Experimenting with Audio Message Playback Controls
Since earlier this year, WhatsApp has been beta-testing the ability to alter the pace of playback of voice notes, and the same feature is now being rolled out to audio messages.
When you send a voice message to a new conversation, it loses its playback speed button, even if the message itself is the same as before. In the absence of this button, you can no longer select between 1.5x and 2x speed.
WhatsApp takes its time launching a new feature, so it may be a while before the function is available on iOS and then on Android..
If you must send voice notes, keep them brief for the time being.