A simple way of making MKV files playable on the PS3 is to convert them into an MP4 file. However, if the source file contains an AC3 5.1 audio stream the sound will be down mixed into a stereo signal when the audio is transcoded into an AAC audio stream.
Fortunately there is a clever solution for this problem. By using the program MKV2VOB, the MKV file is converted into a VOB file.
Even though technically the VOB file requires MPEG-2 video, but the way the PS3 software works, a VOB file with H.264 video and AC3 5.1 audio will playback just fine and provide you with 5.1 channel audio.