By default it seems to return EINVAL.so something like this should do.
Line 953 http://www.freebsd.org/cgi/cvsweb.cgi/src/sys/dev/sound/pcm/dsp.c?annotate=1.62
A+
-- Eric Pouech
Index: dlls/winmm/wineoss/audio.c =================================================================== RCS file: /home/cvs/cvsroot/wine/wine/dlls/winmm/wineoss/audio.c,v retrieving revision 1.81 diff -u -r1.81 audio.c --- dlls/winmm/wineoss/audio.c 17 Mar 2003 00:00:53 -0000 1.81 +++ dlls/winmm/wineoss/audio.c 3 Apr 2003 19:47:39 -0000 @@ -278,7 +278,11 @@ /* turn full duplex on if it has been requested */ if (ossdev->open_access == O_RDWR && ossdev->full_duplex) { rc = ioctl(fd, SNDCTL_DSP_SETDUPLEX, 0); - if (rc != 0) { + /* on *BSD, as full duplex is always enabled by default, this ioctl + * will fail with EINVAL + * so, we don't consider EINVAL an error here + */ + if (rc != 0 && errno != EINVAL) { ERR("ioctl(%s, SNDCTL_DSP_SETDUPLEX) failed (%s)\n", ossdev->dev_name, strerror(errno)); goto error; }