ProjectorsPromethean Projector has no sound

I am having the same issue with my apple tv.  When it is mirroring on the board I have no sound.  I also have the PRM-35 projector.  I need to know what type of cable I need and where I need to connect.  Do I go from the apple tv to the board?  The apple tv only has the optical audio output.  I have tried plugging external speakers into the ipad and using a bluetooth speaker and when those are connected I cannot mirror.  Thank you.