If you use a marker and the audio is quiet/absent, make sure the marker you are wearing is paired as primary.
For C Series:
The best way to check this is simply turn off all the secondary markers and keep the only primary on. After that check the Home screen - your primary marker will always be displayed first.
The primary marker has two green LEDs on its side.
For Swivl CX:
The marker you pair first / turn on first will show up as primary. It always has two green lights next to the Power and the Record buttons and the LED light on its front is white by default.