Edit: As of build 2.25.20.83, this issue has been solved. The Voice Message button now behaves as expected.
PSA: If you use VoiceOver to send voice messages on iOS with WhatsApp build 2.25.20.75 and tend to lock the button, you will find it difficult to send the message. The send button disappears from VoiceOver's consciousness and the only way to have it reappear is to use screen recognition.
If you simply double-tap and hold, speak and then send to release, you will be fine.
I recommend either not updating at this time if you haven't already, or if you have, double-tap and hold for the foreseeable.
Be advised that shutting down WhatsApp completely and reopening the conversation will hide the text box as well as the send button from VoiceOver until screen-recognition is used, so even this is not an easy work-around.
Feel free to pass this on as necessary.