The problem seems top be caused by hardware relays on the card. They do click on and off when initialized and sometimes contact seems to be not made or to break off.

Slow remedy:

Reboot PC. This way whole card gets reinitialized and hopefully starts to work properly.

Quick remedy:

Open "Device Manager" -> "Sound, Video and Game Controllers" -> "ASUS Xonar DX Audio Device" -> Disable Device. And then re-enable device again. You will hear said relays clicking and sound should return to all channels.

The problem seems to be caused by faulty hardware relays on the card. They do click on and off when the card gets initialized and sometimes contact seems to be not made or to break off.

Slow remedy:

1. Reboot PC. This way whole card gets reinitialized (re-clicking all its relays) and starts to work properly.

Quicker remedy:

1. Open "Device Manager" -> "Sound, Video and Game Controllers" -> "ASUS Xonar DX Audio Device" -> "Disable Device". Windows may ask you to reboot if you had sounds played in the last 15sec or so - if it does, say "No" and try again in 10sec or so. (something to do with cache cleanup?).
2. Now re-enable device again. You will hear said relays clicking and sound will return to all channels.