You really need a screen for a projector, even the flattest white walls have some kind of texture that is prominent enough to degrade the image. Luckily screens can be had for quite cheap, at least in comparison to the cost of the projector itself.

But also, if you want high-quality surround sound, then the center speaker needs to be at ear height in front of you. The way home cinema owners arrange that is by putting the speaker behind a screen that has tiny perforations to let the sound through.