If you are using a laptop like me, you may have ran into a problem of the hotkey on your keyboard is not controlling the sound card that you are using when you have multiple sound cards.
It is because XFCE has chosen the HDMI output as my default sound card, so whatever I press, it is just changing the HDMI output but not my headphone/speaker output.
1a) Open “Volume Control” -> “Output device”
1b) Set “Built-in Audio Analogue Stereo” as fallback by clicking the green tick.
1c) Remember the sound card name that you want to set as default, it will be useful in the next step.
2a) Go to “Settings Manager” -> “Settings Editor” -> “xfce4-mixer”
2b) Now, recall the name of your sound card. eg. mine is “”Built-in Audio Analogue Sereo”
2c) Then add “Playback” in front, add “PulseAudioMixer” at the back, and remove all space and ‘-‘ in between, so the result will be PlaybackBuiltinAudioAnalogueStereoPulseAudioMixer
2d) Put this string and replace the original one in “active-card”
2e) Logout and login, then you are done.