So you also got an audio mixer F998 and want to test it on your Linux computer!?
The easier way to have fun with it is connecting its Earphone output to your computer audio microphone input using the P2 cable that come together with your mixer.

Then install this package:

$ sudo apt-get install pavucontrol 

And run this command:

$ pactl load-module module-loopback latency_msec=1

If you want to disable it and return to normal (no microfone sound on speakers) run

$ pactl unload-module $(pactl list short modules | awk '$2 =="module-loopback" { print $1 }' - )

Source: https://askubuntu.com/questions/123798/how-to-hear-my-voice-in-speakers-with-a-mic