Hi there, Bellagaleas8.
I see that, when sending an audio message through the Messages application on your iPhone, it sounds muffled. When you listen to the ones your friend sends you, they sound just fine. You don't believe it's the microphone causing this since you can send an audio message through WhatsApp just without problem. I'm glad to help with this.
If the microphones on your iPhone, iPad, and iPod touch aren't working -- Since it's unknown which microphone WhatsApp application uses, please test the three microphones on your iOS device.
Set up Messages on iPhone -- If, as you suspected, it's not a microphone issue, please turn off iMessages by going to Settings > Messages, then restart your iOS device: Restart your iPhone (if you’re using a different iOS device, there’ s a link near the top if you need) -- Once you're back to the Home screen, turn on iMessages again and test the issue.
Let me know how that goes, and take care!