I have a recent (AMD Athlon II X4 645 based) PC. Although the motherboard has integrated sound, I use an ancient-but-still-pretty-good Creative Soundblaster Audigy Platinum Ex. I dual boot Windows 7 and Windows XP at the moment.
Today, about an hour ago, using Windows 7, I noticed that I wasn't getting any sound when I started a game of Oolite. I know the sound was working around an hour before that. Not sure if I rebooted in-between or not, but I had the speakers switched off for a lot of that time, so didn't notice exactly when the noises stopped.
At that point, I had only used Windows 7 all day (I haven't allowed XP on the internet for a few years now). There were no driver updates in this time, nor any settings changes. The most recent drivers for this card were released in 2010 IIRC. About all I've done today is browse a few web sites, watch a couple of videos in VLC, and update some backups on an external hard drive.
I checked the speakers and cable, and the state of the sound card and drive in the device manager, and of course the volume control. Everything seemed fine. I thought maybe the old card had failed.
But when I rebooted from Windows 7 to XP as a final check, all the sound was fine. I rebooted back to Windows 7 again and the sound was dead again.
The fix was simple enough in the end. The driver executable for the sound card (SBAX_PCDRV_LB_2_18_0017.exe) offers to remove existing drivers if they're found, so I ran it once to remove the drivers, rebooted, then ran it again to reinstall them and rebooted again. All sound is now fine.
But this isn't very satisfying. Obviously reinstalling (the exact same version of) the driver has reset some setting that had got broken, but ideally I'd like to know what the cause was so that I can prevent it happening again.
The most recent drivers for this soundcard have turned out to be a bit buggy. I only got this particular glitch a couple of times, but I found a much bigger problem when I tried to use a microphone in Windows 7 64-bit. Basically, chunks were missing from the audio waveform, giving the impression of one of three or four buffers being wiped to all zeros every time it's used, which is several times per second recording 48KHz.
My final solution was to give up on that soundcard. I'm currently mostly using motherboard HD audio, though I also have a USB SoundBlaster Audigy 2 NX which is slightly less obsolete and doesn't have the same driver issues (but also doesn't have MIDI or firewire ports).