If you cannot be heard in Fortnite Chat on Xbox One, but your headset's mic is working well with other devices or in Xbox Party Chat, please do the following:
In Fortnite, access the Main Menu. Then, go to Settings >> Speaker icon.
Turn Voice Chat from ON to OFF.
Turn Voice Chat Method from Open Mic to Push-To-Talk.
Back out of the Settings Menu, to ensure those settings save. Then, open the Main Menu, and go back to Settings >> Speaker icon.
Turn the Voice Chat from OFF back to ON.
Turn the Voice Chat Method from Push-To-Talk back to Open Mic.
Save those settings, and then exit the game completely.
Once the game is closed completely, start the game up again, and test the chat out. Can you be heard clearly?
If this issue persists, please contact our Support team for additional assistance.