Whenever you join a friends party while in-game, you can only hear them if you have the party "snapped" if you unsnap it, you wont be able to talk to your friends unless you exit the game completly. Something Bungie should know about and fix.
English
#feedback
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If you are having extensive trouble, and it is getting annoying, go to game chat. Game chat in Destiny does not work like, say Halo, where everyone is talking at once. Only your fire team can hear you, if you are not in one, you are sitting forever alone in silence (that was a joke).