BGMCHK in SmileBasic V1 is identical to that in V2.
BGMCHK([track number]) is a system function which indicates whether music is currently playing on the specified track. The track number must be between 0 and 7 inclusive (if the number is omitted, track 0 is used). The function returns
TRUE if that track currently has music playing on it, or
No information on SmileBasic V3.