Sound doesn't work on system startup

[Sorphius]

Twice now in the past week, I've started up my computer to find that my speakers aren't playing any sound despite everything working fine the last time I shut it down.

The sound device is set to ASUS Xonar DX (sound card), and my speakers are plugged in and turned on with the volume at what should be a normal level.

When I open the equalizer while music or a video is playing, it shows the bars moving up and down as though sound is playing, there just isn't anything coming through my speakers.

Headphones work fine, and I can get sound from the speakers built into my monitor, but my main desktop speakers won't do anything.

When this happened the other day the problem resolved itself after I rebooted the computer. I haven't tried that yet today (since it just happened), but I suspect that will probably fix it again.

Any ideas what may be the problem?

*Edit: Just restarted and have sound. Still no clue why it's happening though.

[Sorphius]

Here you go.