Whenever I receive a voice note through iMessage it only plays through the small speaker as if I’m on the phone not as if it’s on loud speaker but doesn’t do this when I receive them on WhatsApp. How do I change it to play on loud speaker through iMessage?
To my knowledge, it is designed to be played via the earpiece, not the speaker. Read the section in this support document about sending an audio message and listening or replying. As you see it says to raise the phone to your earl. Send and receive photos, videos, and audio messages on iPhone - Apple Support
Audio messages
