We are aware that the audio feedback for some users (on iPhone devices) has a robotic voice after the recent iOS 12 update. This is due to a change in Siri's accent settings and our team is looking into the issue. Currently, we do not have the ability to revert back to the original Siri voice within Strava and are working on a fix.
We appreciate your patience. You can "Follow" this Known Issue for email updates.