This Universal SysEx message adjusts a device's master volume. Remember that in a multitimbral device, the Volume controller messages are used to control the volumes of the individual Parts. So, we need some message to control Master Volume. Here it is.
0xF0 SysEx
0x7F Realtime
0x7F The SysEx channel. Could be from 0x00 to 0x7F.
Here we set it to "disregard channel".
0x04 Sub-ID -- Device Control
0x01 Sub-ID2 -- Master Volume
0xLL Bits 0 to 6 of a 14-bit volume
0xMM Bits 7 to 13 of a 14-bit volume
0xF7 End of SysEx