This MIDI player setting Common volume can be turned on or off by checking or unchecking the menu item.
Adjusting volume level is done by changing volume and expression controllers percentually up or down so that the song maximum volume level is close to common volume level. It does not change note velocities because that could influence the sound.
If the songs are generally too loud or too silent after adjusting to common level then you should adjust your speaker volume or keyboard main volume slider.
In some cases it is not possible to reach a level close to common level, e.g. if one channel already has maximum level then it can't be increased further.
Default common volume level is 1000000, this value is written into gnmidi.ini file:
If you want change this value then use only values between 300000 and 1700000, values lower or higher lead to extreme use of volume changes which might
cause bad arrangement. If the setting is checked then it is turned on and starting MIDI player (play or play extern) will produce a temporary MIDI file for playing that has adjusted volume. In some cases volume is already at limit and can't be adjusted to the wanted level (e.g. if expression controls or note velocities prevent from getting higher volume).