After starting the first session of the SOUND.EXE program, the speaker will produce some more or less beautiful noise. Starting a new VDM session with SOUND.EXE will prevent the first session's SOUND.EXE from executing because it is switched to the background (the speaker system is not virtualized!).
The second session cannot access the speaker because it is blocked until it is granted access to the speaker.
You cannot terminate the program by pressing "Enter" because the "PLAY" instruction (refer to the program listing in Figure "SOUND.BAS Source Code") could not be completed.
If the first session is switched back to the foreground, it resumes execution. Pressing "Enter" ends session 1 and now session 2 has access to the speaker and begins to play the music.