Hi jintiy,
Thank you for giving us the chance to assist you with your iPhone 11. You can count on us to do our best to assist you or get you pointed in the right direction.
We understand that your device is not able to capture the first 5 seconds of your audio message. While it isn't inherently assured that an update will resolve the issue, updating to the latest version of iOS for your device can help isolate potential software issues. Kindly secure a backup and update.
How to back up your iPhone, iPad, and iPod touch
Update your iPhone, iPad, or iPod touch
If the device is already up to date, let us know if you are experiencing a similar issue when recording a video and Voice Memo.
Please keep us posted.
Take care!