Hello. I've found a workaround for this problem. (latest -hg snapshot)
Testing again with osstest, one test didn't blast my ears to bits.
/dev/oss/oss_hdaudio0/spdout0 (audio engine 5): HD Audio play spdif-out
- Performing audio playback test...
After I symlinked /dev/oss/oss_hdaudio0/spdout0 to /dev/oss/oss_hdaudio0/pcm0 the sound worked perfectly. Maybe this is a hint for the OSS developers to fix the bug.
It only works if I disable input-mix.
this workaround disables vmix