Music will not play to connected speaker

I am trying to play music for my son's baseball game via a speaker attached to my Android via an adaptor cord. When attached the music continues to play from my phone's speaker. I verified that if I plug in a headset, the sound does come through the headset instead of the phone speaker. There was also another person who tested his Android and it worked fine with the speaker. Any thoughts on what might be causing this and how to address?