Improperly encoded audio happens a fair bit on PBS and CBC because they always send out a DD5.1 signal, even when the original material only contains 2 channels. Most broadcasters send out DD2.0 or DD5.1/5.0. With the improperly encoded audio, you get the LF & RF channel and sometimes, if they really screw up you get no dialogue due to the missing center. See the following FAQ, item 1. Workarounds are also discussed in the FAQ.
http://www.digitalhome.ca/forum/showthread.php?t=17870
http://www.digitalhome.ca/forum/showthread.php?t=17870