You may want
to look into ALSA (
http://www.alsa-project.org) which is
the new Linux sound architecture. Also, it would probably be good to
make sure your audio card is actually capable of full duplex and that
this feature really does work in Linux (with the drivers you decide to
code for). I can't think of any quick and easy programs to test full
duplex (anyone else have any ideas?). Good luck.
I think what you could do in ALSA is to make a "loopback" connection
from your soundcard's Line Out back into its Line In, then playback some
.wav or .mp3 with e.g. aplay and simultaneously (try to) use arecord(*)
to record whatever comes back in on Line In.
this isn't full duplex. its two independent processes using one
direction, which doesn't actually test the driver in the same way as a
single process.
JACK (jackd) runs full duplex by default, and so could be considered a
test app.
--p