With Safari 11 and up, a new feature has been implemented that allows users to control websites' ability for audio and video to play automatically. Unfortunately, this feature can interfere with Slacker's ability to play music continuously. In order to allow Slacker to play music continuously, you will need to adjust the option within Safari.
Note: You'll need to have the Slacker website open in a Safari window while doing this.
1.) From the Safari menu, select Preferences.
2.) From the Preferences window, select Websites.
3.) Within the Websites section, select Auto-Play.
4.) Within the Auto-Play section, find slacker.com and change the setting to Allow All Auto-Play.