1.4.2 Audio Control: If any audio on a Web page plays automatically for more than 3 seconds, either a mechanism is available to pause or stop the audio, or a mechanism is available to control audio volume independently from the overall system volume level. (Level A)
Note: Since any content that does not meet this success criterion can interfere with a user’s ability to use the whole page, all content on the Web page (whether or not it is used to meet other success criteria) must meet this success criterion.
Automatic playing content distracts users, especially cognitive, screen reader users & will hinder in performing their tasks effectively. If you can avoid, don’t play audio automatically. While there is an exception that audio that is less than 3seconds can be added to a web page & an exception that providing a pause/stop or volume control will pass this success criterion we still recommend not to play audio automatically on the web page.
While many may argue that the system could be lowered to prevent distractions, that solution would not solve the problem. It is mainly because the assistive technologies like screen reader volume is based on the system volume and the volume of the automatically playing audio content must be independent of the system volume.
The best example of playing audio automatically is Youtube videos & other video sharing sites.