Looking beyond ATSC, the possibilities for carrying alternate audio programs certainly still exist but with some additional constraints.
Just like ATSC, newer formats will use video and audio compression and a multiplexing scheme. The major difference is the type of compression employed. New video compression formats such as H.264 or Microsoft VC-1 allow far greater data reduction for a given picture quality. This is critical for delivery systems such as IPTV that can be data bandwidth limited. If the overall available data rate is restricted, then a tradeoff between quality and amount of content becomes reality. Delivering an HD picture and many channels of audio in a 19 Mbps ATSC transport stream is easier than doing the same thing in an IPTV environment.
Suddenly, alternate audio versions loom very large in comparison to accompanying video. Where two or three 384 or 448 kbps 5.1 channel programs can easily exist in an ATSC transport stream, as the available data bandwidth drops, so must the audio content. Either fewer audio programs can be carried, or the data rate of each program must be reduced. New audio techniques are becoming available some 15 years after the release of Dolby Digital (AC-3). Of the two leading new formats, one is based on Dolby Digital and one is based on the AAC audio coding system. Both are starting to be introduced by broadcasters and both address issues of getting additional audio programs sent through paths of decreasing size.
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