I'd check the task manager to see if they were still physically running. Once in a while, a certain program's icon will disappear from my system tray, but the program will still be running fine; I've seen this with ZoneAlarm and McAffe anti-virus, as well as the Creative volume control icon. If you're unsure, just restart.
If the computer has been on the whole time and they are showing in taskman, it just means explorer crashed.
Top Gun, "Creative volume control"? If you want to improve your systems performance a lot, I recommend you remove everything from Creative-ways to slow down your computer ASAP.
I actually really like the Creative software; there's a few applications in there that I use somewhat frequently, plus it enables easy sound recording of whatever is coming out of the speakers. There are probably other programs out there that do all of this, but I'm willing to sacrifice a little performance to keep what I have.
I didn't say a little performance, I said a lot Creative's drivers and software is some of the worst bloatware I've ever seen, but it is your computer so what works for you works for you.
vid card drivers often affect icons, since loading up 76.10 dets desktop icons take longer to refresh and the taskbar ones tend to vanish now n then. but the faster performance i get out of them more than makes up for it.