No, not everyone was mummified. Simple reason being, mummification was a long and ultimately costly process. Therefore, it's not surprising that most of the mummified remains we've ever found are either royal or of some high political, military or religious official.

Those who weren't mummified were usually buried in a shallow grave somewhere in the desert with their belongings, just like a normal Egyptian burial, just far less lavish. This was actually very successful at preserving the body, due to the dry climate and all.

However, a note on those you define as 'slaves' and such: being a slave didn't necessarily make you less of a rightful human being, it was merely your position in life. If my memory serves me correctly, many of the Old Kingdom Pharaohs actually chose to be buried with their officials and 'servants'. Therefore, they were also mummified. So there are exceptions to what I said at the beginning, but these are few and far between.