In General such a problem. Connecting the usb speakers (the same sound is still 5.1), I want to make them the default device, put in the panel via hd audio output SPDIF or SPDIF2 — no result.
Put in AIMP3 output these speakers — all OK, works. But from the control panel does not want. It was my decision.
Maybe you can do something to make the app X put the output sound to USB speakers and not on a 5.1?
Via HD Audio it is likely that your normal sound card, not the one that you connect via USB. SPDIF to USB has nothing, so nothing worked. To force app X to play through the speakers you can only if this application supports a variety of playback devices (e.g. VLC or for example CS:Source). If you are on 5.1 speakers watching movies it makes sense to put the default USB and configure the player to playback via HD Audio.