With the update to Safari 11, a new feature has been implemented that allows users to control websites' ability for audio and video to auto-play. 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 want 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.