I know this was a feature request by some here. It looks like Tesla has listened. According to greentheonly on the Tesla subreddit, 2018.26 Software Megathread • r/teslamotors, the release notes for 2018.26 contain this release note text for the Model 3:
Currently no known Model 3's in the wild have any version of 2018.26.x currently installed.
Lock Confirmation Sound
Model 3 is now able to emit confirmation sounds when locked, unlocked, or if there is a mislock (for example, if a door is not fully closed). To enable this feature, tap Controls -> Locks -> Settings -> Lock Confirmation Sound.
greentheonly has been known to somehow get a hold of the actual raw software download and able to grab out the release note text from it. He or she was able to grab the full Model 3 release notes for the 2018.24.x version before anyone else even had a screenshot, so I take them as a reliable source.Model 3 is now able to emit confirmation sounds when locked, unlocked, or if there is a mislock (for example, if a door is not fully closed). To enable this feature, tap Controls -> Locks -> Settings -> Lock Confirmation Sound.
Currently no known Model 3's in the wild have any version of 2018.26.x currently installed.