An audio selector label allows you to listen to whatever had been plumbed into it. Selectors normally exist for COM 1/2/3, NAV 1/2, DME 1/2, ADF 1/2, HF, intercom, PA etc. You listen to as much or as little as you need to. This allows two active frequencies (if you wish) and however many standbys as your systems allow.
When you have multiple FMS boxes you typically find they speak to each other. Some systems call this "Crosstalk". So an input on one box updates the other.