Why /dev/dsp can get into "Device busy" state?

[Yuri]

At some point, after killing audio playing app with Ctrl-C, I am unable 
to play sound again with the same app:
Can't open audio device /dev/dsp: Device busy
But after closing virtual box windows guest, sound is working again.

What bothers me, why audio device can even become busy? Isn't it 
supposed to mix all inputs no matter how many? It usually works fine 
with many apps playing sound at the same time.
What exactly is the condition that causes sound device to become busy?
Driver in use is snd_es137x.

fstat /dev/dsp doesn't show who opened sound device for some reason.

Yuri