|"...I have lost that icon on the task bar. I haven't seen it there for a long time..."|
Could you at least tell us if/what you have tried to get that Volume Icon back, and with what results? You can still access the volume controls through the Control panel/Sounds and Audio Devices -- Volume (click on Advanced to Mute/Un-Mute individual devices).
You know, of course, that the icon will "disappear" after a while as per the default "Hide inactive icons", but can simply be made to "Show hidden icons".
Is there any Exclamation alerts in Device Manager/Hardware regarding related devices?