Stand by = Sleep (Hibernate for Vista)
No. There are four power states.
The computer is on, you are using it.
The computer is off.
the computer is on sleep/standby (low power mode, HD's powered down, most peripherals powered down)
Hibernate. The computer saves all RAM to disk and shuts off.
The only plausible scenario here is that your computer is NOT Off, it's in sleep/standby mode (NOT hibernate, either) because otherwise only the power switch would turn it on.
When I stand-by my computer and start it after, the sound doesn't work. I need to reset the sound for it work again Why?
Fixed the above quote (stand by to the more clear stand-by).
The answer? your sound hardware/driver isn't compatible with power saving mode.