If you are using a TeensyUSB, there is a [known bug](https://github.com/qmk/qmk_firmware/issues/164) in which the hardware reset button prevents the RESET key from working. Unplugging the keyboard and plugging it back in should resolve the problem.
## Special Extra key doesn't work(System, Audio control keys)
You need to define `EXTRAKEY_ENABLE` in `rules.mk` to use them in QMK.