Hi
Install CCleaner, and look in Tools Startup.
There's a free version.
Look for any program that has a connection with Sound and try turning it off.
There are other ways to do this but CCleaner makes it very easy to disable and re-enable start up items, and everyone should have it anyway.
If you don't see anything there that relates to your sound, try uninstalling your sound card in Device Manager and then reboot.
Windows will reinstall your sound device and hopefully set it back to the default settings.
If that doesn't work I'd try updating the drivers for it, and and hope that will reset it to the normal configuration.
I don't know of anyplace in properties or setup where there is an option to mute on start up, so I don't know...