Your wireless headset microphone has mic ducking. The mic has priority over the music- which means that the music will drop down, slightly, so that the mic will be heard clearly.
To maximize the effectiveness of the mic ducking, please keep the mic volume close to 100% and raise the music up so it rests just under the mic volume overall. Then the mic will always be heard and the music will have the correct energy level for the fitness experience.
As a point of reference, if the mic is set to 10, then the music volume should be around 8. (ratio of 10:8)