How to :
- To send voice messages first update your Facebook Messenger app available from Android and iOS stores.
- Open Facebook Messenger app and click Compose button
- Tap the + button on the left side of compose window, then click the "Record Video" option next to the Image search.
- Record your voice message which you want to send
- After recording completed tap Send button
Your friend also should have latest version of Facebook Messenger app to receive your voice message, otherwise he can't receive your voice messages.