[ecasound] dsp busy!

Hi everybody,
I have an mp3 file and I'd like to play piano on it and to record by a
microphone the result.
I tried this:

ecasound -b:256 -r -a:1 -i base.mp3 -o /dev/dsp -a:2 -i /dev/dsp -o
new_track.wav

but it seems I'm not able to open the dsp device in duplex. If I try just
to play a file i have no problem but in this case the /dev/dsp device
remain locked and I don't understand how to unlock it. I'm forced to
reboot the system.
The /dev/dsp remain busy.
I also try to use the other dsp* devices but they are unavailable.

thanks

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