I've bought an Asus Xonar D1 sound card a year ago. Since then I've been enjoying the sound quality and being plagued by it's bugs. Some were solved by switching to 3rd-party drivers, but one bug persisted:
I'm using the card in 2 channel mode (stereo). Sometimes after PC boots up one of the channels would be silent. Rarely that would also happen mid-work, one channel would suddenly become mute. So the sound would be playing only through the other channel. Changing sound properties (volume, balance) or soundcard drivers features (DG and such) would not help.
The bug persisted with official and 3rd-party drivers. Jacks and sound-system are not to blame (swapping all of those those did not help). Which lead me to believe that's a hardware problem (see the answer).