Your Voice Memos app can go missing from your home screen for 2 reasons: you accidentally deleted it, or it's just hidden somewhere somehow.
In case you accidentally delete the Voice Memos app, simply restore the app from App Store. If the App Store indicates that the app has already been installed, then it's still in your iPhone. Use the spotlight search to find it. Or you can alternatively reset the home screen to the default layout by going to Settings - General - Reset - Reset Home Screen Layout. Then your Voice Memo app icon will show up in the Utilities folder again.