Hey Zor , long time no see here , and actualy i dont know is this good or bad
Anyway , culd it be some bug with volume set , actualy dunno how to check it .
Other . uninstall all soft/driver for sound card, and use some registry editor ( i use Regedit ) and manualy search for left over of the soft , this probale wont help , but some times i was able to make things work (with the search option not totaly manualy ). As you say its not HW problem , something in soft/driver got broken .
Or , best if ther is some software that is desingned to clean files of you SC . But that probable dont exist , and anyway the problem me be deper.