The maximum volume level of all logical devices in the system are controlled by the Volume Control application supplied with OS/2 multimedia. The Volume Control application sets a scale by which all subsequent volume commands to the ampmix device are based. For instance, if the Volume Control sets the master volume at 50%, then all volume levels are cut in half.
Some devices may only support two levels of volume (on/off). These devices are off when the master volume is set to zero, and on at any other value.
Note: While the MCI_MASTERAUDIO message can be sent by any application, only the master volume application or a replacement should utilize this message to set the master volume. Master volume should only be controlled at the discretion of an end user as implemented in the Volume Control application. A parameter of the MCI_MASTERAUDIO message allows an application or a media driver to query the master volume level.