Email clients (ie Gmail, Apple Mail, Outlook, etc) do not support audio for Autoplay Video Email.
When you open a video email the video that plays immediately is referred to as the Autoplay Video. The simple answer is that NO email client supports any audio for these Autoplay videos during play.
There is also very little demand for sound to occur when the email is opened. This is similar to the 'click to unmute' or 'click to watch' videos you see in social media, on websites, etc.
Playable does offer an optional feature, under the Click Action menu to play an HD video with quality sound, once the autoplay video is clicked by the user. In Apple Mail, this video will play within the email, while other email clients will play this video in a browser tab.