Originally, the PLAY-function from QBasic supports MF and MB flags:
 
Mode commands:
MF -- Plays music in foreground.
MB -- Plays music in background.
 
MF is the default, MB could be added, so it will be played in a seperate thread. This could be combined with an OnPlay event. Originally this is done like:
 
ON PLAY(queuelimit%) GOSUB line
 
If %queuelimit is 3, it will trigger when there are 3 notes left to play. I propose to use the nativeeventhandler and add the amount of remaining bytes to process as data2 parameter, so a simular working can be provided.