Hello
How can transfer data over sound card between two computer ?

I believe that comes under 'pointless hacks'. Possible, but why bother?

If you want to transfer data, use a network card, not a sound card.

I used to do this when I was playing around with creating a voip system.

I think the file /dev/dsp is reliant on oss:
http://en.wikipedia.org/wiki/Open_Sound_System#API

I think you can mknod /dev/DSP while running alsa.

I agree it's "pointless" but in retrospection I've done a lot of pointless things with computers. This sounds like fun.

http://awesomegeekblog.blogspot.com/...ound-card.html

"AnonymousApril 23, 2009 at 9:43 PM
Clever... but were you aware that someone already did it and you might already have the drivers on the machines? Linux's AX.25 drivers include a soundblaster interface. AX.25 uses frequency-shift keying (two different tones for 1 and 0), and transmits around 1200bps.

It is intended to be connected to an amateur radio, but can be hooked up from line-in to line-out if you want."

It is possible. Transfer speeds will be on the order of 1200BPS.

http://www.whence.com/minimodem/