Here are a few things to consider when mixing dialog (and that includes announcers) for 5.1 surround sound for broadcast.
Dialog should be sent to the center channel.
Be aware that the center channel in the Lt/Rt downmix is reduced by 3 dB. If a viewer listens in only stereo or mono, the dialog/announce level may be too low compared with the rest of the elements in the mix, and may even be inaudible some or all of the time. Because of this, it may be necessary to boost the level of the announcers in the center channel, while making sure that the left and right levels don't become high enough to drown them out.
If the channel mode metadata parameter sent with the 5.1 program is incorrectly set to 2/0 instead of 3/2, this can present problems for a viewer with an AC-3 decoder that is expecting to receive the 5.1 discrete surround mix. This is because 2/0 directs the AC-3 decoder to output only two channels, the left and right; it doesn't mix in anything from the center channel.
(Note that this kind of problem should be corrected at the source of the metadata error, but it may not be quickly discovered or diagnosed. With metadata analyzers now becoming readily available, hopefully this will be less of an issue.)
So, try adding in some dialog/announce equally in the left and right channels, but perhaps a bit lower than in the center channel. Of course, make sure all channels are in phase and in polarity with each other, and check the Lt/Rt downmix for proper balance.
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