Hello Zoeyuen,
It sounds like when you play a WhatsApp voice message, sometimes the sound is fuzzy.
Here are some resources and troubleshooting steps that can help:
Review If you hear no sound or distorted sound from your iPhone, iPad, or iPod touch speaker.
Test if this is happening when connected to a particular Wi-Fi network or cellular. You can also check to see if it is happening in a particular location to see if it's a network issue.
See if this is isolated to just WhatsApp voice messages by testing out a phone call or FaceTime call using the receiver when this happens. If it continues with just this app and it's not a network issue see If an app on your iPhone or iPad stops responding, closes unexpectedly, or won’t open as the app is not working as expected. Reach out and contact the app developer for assistance if it continues How to contact an app developer.
Hope this information helps.
Have a good day.