Do you have auto-volume set in the audio menu?
Also, if you think this is an issue beyond settings, and you're a U.S. customer, register your product online, and you may qualify for an additional three months of warranty service. After registering, you can call 800-SAMSUNG if you think there is something wrong with your set.
Of course, that's all up to the service department, but it may help in your particular situation.
After 14 months of defaulting to whatever the video mode was previously used before shutdown, usually cable STB, my LN46A LCD TV now always powers up in the TV tuner mode at full loud volume. I can use the Source button to select whatever input I want so I can still watch the tv. It is annoying however to not only have to change the video source, but quickly turn down the volume from full loud when I power up the set. I have tried all factory resets, etc to no avail. Any ideas?