The main noticeable feature is an additional animation bringing the marker back to the beginning at the end of a voice message, according to WhatsApp update tracker WEBetaInfo.
In addition, one common feature in WhatsApp is to mark "Read Receipts" as disabled, meaning that the user one is chatting with would not be able to see if their messages were read or received; in other words, they would not be able to see the famed blue checkmarks.
With the new update, this feature will also apply itself to voice messages, meaning that someone with this feature disabled would also block those they converse with from seeing if they heard a voice message sent to them.