Why am I not able to adjust the sound volume, or switch to the next/previous audio track?

The interval between each press may be too long when using the multi-function button.

By pressing the multi-function button rapidly, within an interval of about 0.4 seconds, will make the action you are executing to be recognized as a continuous function.

How to operate

 [1] multi-function button

Play/pause: Press the multi-function button lightly.
Next song: Press the multi-function button 2 times consecutively.
Previous song: Press the multi-function button 3 times consecutively.
Increase volume: Press the multi-function button 2 times consecutively, holding the button down at the second push.
Decrease volume: Press the multi-function button 3 times consecutively, holding the button down at the third push.