I find all sounds my computer makes a bother. I have managed to turn off almost everything, but the "change state" between "on battery" and "on power" still remains.
How can I turn it off?
If you have the Lenovo Power Manager utility installed, go into 'Global Power Settings' and uncheck 'Beep when power state changes'. (This is on my T60p, Win7 32-bit with Power Manager 3.05)
Depends if you're referring to a system beep or a sound from the sound card. Either way, both can be disabled in device manager.
For your sound card:
I have a T41, but on that computer, you can switch the sound on / off in the BIOS setup.
I have a Lenovo ThinkPad T400, which I've owned for about a year. A while back, I noticed a problem in which the LED backlight would not always turn on when booting from battery. Usually, if I hold the power button down until the BIOS screen shows up, the backlight turns, but sometimes even that doesn't work. If I just press the power button and let it go right away, the backlight usually doesn't turn on.
This happens before the OS (I dual-boot Linux and Windows) gets a chance to boot - the BIOS screen itself is displayed without the backlight if it fails to turn on, so the problem isn't at the OS level.
Best way is contact Lenovo Support or apply warranty for your ThinkPad.
Because it's look like HW problem.