How does a 5.1 channel program get reproduced by consumer gear that is only capable of stereo or mono? Through a metadata controlled process called downmixing.
Parameters such as Center Mix Level (cmixlev) and Surround Mix Level (smixlev) determine how much of each of these signals is included in the two-channel downmix. The default for each of these is -3 dB (i.e. the signal multiplied by 0.707). While mathematically these are the proper values, it is critically important to listen to the results through a stereo decoder and a mono decoder. Professional Dolby Digital (AC-3) decoders will allow monitoring in all modes and simplify the process.
Remember, that although 5.1 channel surround sound continues to rapidly increase in popularity, the majority of viewers will still hear the audio as stereo or even mono. This is mostly due to cable and satellite carriage of programming, which use set-top boxes that in addition to a 5.1 channel encoded output, will provide audio on stereo RCA connectors and/or RF remodulated on the venerable channel 3/4 output for the widest compatibility.
The default levels are also the maximum that these downmix parameters can be set for, and each can be lowered to -4.5 dB or -6 dB. If during monitoring it is determined that the center dialogue is difficult to understand, since it is not possible to raise the level further, try lowering the surround mix level. It may be that effects contained in the surround channels are overpowering the center channel audio.
If the metadata values being provided to you from external sources do not sound correct and you cannot change them, make sure to complain to the provider and explain the problem--they might not understand that these things need to be checked.
