Bad voice message quality in iMessage only

Hi Yen ting Huang,

Thank you for using Apple Support Communities. I understand you have a question related to the audio quality in iMessage recordings. I'd be happy to assist.

If you have already restarted your iPhone and your issue is persisting you'll want to take a look at this article related to microphone quality on iPhone: If the microphones on your iPhone aren't working

Specifically, take a look at this section:

""If people can't hear you on phone or FaceTime calls

1. Open the Voice Memos app. 
2. Tap the Record button , speak at the bottom of your phone, then stop the recording. 
3. When you play back the memo, you should hear your voice clearly.
4. If you don't hear your voice clearly, contact Apple Support."

Enjoy your day.