The MMX 300 and Custom Game are pure stereo headsets, that means that they have one speaker on each side. However, surround sound can nowadays. ideally be simulated via the so-called "head related transfer function", so that a realistic surround sound image can also be achiedved with stereo headsets.
The soundcard of the PC must have appropriate technologies, such as "Dolby Headphones" to simulate surround sound.