BGMGETV is not a function in SmileBasic V1.
BGMGETV is a system function which takes two parameters, a track number (0 to 7 inclusive) and an MML variable number (also 0 to 7 inclusive).
If MML music is playing on the selected track, the function returns the value of the corresponding variable. For instance, after the code
BGMSET 128,"$2=90[R]":BGMPLAY 5,128, the function
BGMGETV(5,2) will give the value
90. Uninitialized MML variables have the value -1, and if the specified track is not playing an MML string, the function will return -1.
See also BGMSETV (Command)
No information on SmileBasic V3.