Audio accounts for 50 percent of the cinema experience, and a surround sound system can create a complete audio environment around you.
A good 5.1-channel system will give you a full surround sound experience. Most DVD and Blu-ray™ media, some Super Audio CDs (SACDs), broadcast TV, and many streaming sources are in 5.1-channel format.
Going to 7.1 channels improves the directionality of sound effects and adds to the audio ambience of 3D. Some Blu-ray Disc™ and premium streaming sources feature 7.1. (They will sound fine on a 5.1 system, too.)
A 9.1-channel system adds front height speakers to take advantage of Dolby® Pro Logic® IIz, which derives height information from the signal.
A Dolby Atmos system creates audio "objects" and overhead sound to create a true multidimensional audio experience.