V1[]
BGMVOL is not a command in SmileBasic V1.
V2[]
BGMVOL is a command for controlling the volume of the sounds playing on any of the 8 tracks. Each of its numerical parameters is rounded down if it is not an integer.
If BGMVOL has only one parameter, track 0 is used and the parameter specifies a volume. If BGMVOL has two parameters, the first specifies the track number and the second is the volume.
The track number must be between 0 and 7 inclusive. The volume must be between 0 and 127 inclusive. The command sets the music playing on the specified track to the specified volume.
V3[]
No information on SmileBasic V3.