V1[]
BGMSETV is not a command in SmileBasic V1.
V2[]
BGMSETV is a command which changes MML variables. It takes three numerical parameters, each of which is rounded down if it is not an integer; a track number (0 to 7 inclusive), a variable number (0 to 7 inclusive), and a value to write to the variable (0 to 255 inclusive).
For example, after executing BGMSET 128,"$2=60[RRRRN$2]":BGMPLAY 5,128, the system will regularly play a note. After BGMSETV 5,2,80, the note will change.
See also BGMGETV (Function).
V3[]
No information on SmileBasic V3.