05:06.0 Multimedia audio controller: VIA Technologies Inc. VT1720/24 [Envy24PT/HT] PCI Multi-Channel Audio Controller (rev 01)
It might be a bug in the driver.
Open Sound System v4.2 build 2005 has been announced for Linux (x86/amd64), Solaris (x86, amd64, sparc), FreeBSD 8 (x86, amd64), UnixWare, and Open Server.
* Added support for Envy24HT PCIe
Perhaps, Cesium may explain the problem.
EDIT: You may try the pcm slider.
Another option is to disable vmix (it is a kind of PulseAudio).
It is very probable that your soundcard has HW mixer. In this case, you do not need vmix.