Perhaps if you could share a bit of code to show what is happening.
The SOUND command will halt all program execution until it is finished playing. In QuickBasic it was possible to use SOUND 0,0 to stop sound, however with QB64 this will not work.
With the PLAY command it also be hard to control when it stops unless you control each note given to it one by one in a loop. Very tedious and not worth it.
I suggest making your sounds utilizing .WAV, .MP3, or .OGG files. This way you can start them with _SNDPLAY and stop them any time you like with _SNDSTOP. The new QB64 sound commands are far superior to anything that QuickBasic had to offer.