sound is off when display is off

[tommer2]

when I do this :
0 start my macbook on battery (unplugged)
1 start vlc
2 play music Repeat All
3 do nothing for 2 minutes
4 my macbook turns its display off
5 the song that is playing play until the end of the song
6 VLC starts a new song, but without sound

anybody knows why?

when I press any key, my macbook's display turns on again, but still no sound.
the only 2 ways to get sound in VLC are : jump to next song or start and stop current song.

i reinstalled VLC Mediaplayer : same problem