If you cannot be heard in Fortnite Chat, but your headset's mic is working well with other devices (or in Xbox Party Chat, if used with an Xbox console), please do the following. Please note these steps can be used for Xbox consoles, Nintendo Switch consoles, and PCs.
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.