Running zone diagnostics will check the connections on your speakers. To do so, navigate to the Zone Diagnostics page in the Settings page. Once there, press the 'Run Zone Test' button.
Once the tests complete, check to see if there are any warnings by the zones with the speakers that you are trying to play to. To view the exact warnings for each zone, click on the zones with the 'warning' label next to them.
Click on the 'Warning' button next to the zone that is having the issue. The specific warning messages can be found by clicking 'see more' in the popup.