WhatsApp lets you change the speed of playback of voice notes since earlier this year and it’s now beta-testing the same speed control for audio messages.
An audio message is a voice note, which you’ve forwarded to another chat, and in that forwarding process, it loses its playback speed button, despite remaining the same exact message. The button, which allows you to choose 1.5x or 2x speed, disappears.
WhatsApp does its due diligence when it comes to introducing a feature, so it may take some time before the feature lands on iOS and subsequently on Android.
In the meantime maybe only send quick, short voice notes.