rbultje, what you mean by 5.1 surround is probably AC-3, which is handled perfectly by liba52 (AKA a52dec, ac3dec). The downsampling is also standardized, and is done in such a way, that the two-channel "stereo" output can be decoded by any decade old Dolby Surround receiver to get to a bearable imitation of 5.1. That is, Left, Center, Right, and one Surround, going to two rear speakers. The .1 is just a low-pass filter away :)

With the majority of consumer gear, it is not easily said that discrete 5.1 is better or worse from Dolby Surround-style faking of it... And the latter is not really different from a good stereo amplifier's sound, if you are listening right at the "sweet spot". Dolby Surround largely does what your ears are doing. Yes, you can find some missing details with advanced systems, but you decide how much does it worth...

Interestingly, the reason most older DVDs had 5.1ch and 2ch "mixes" of the sound is that one is a bit more "optimized" for Dolby Surround decoding.. Nowadays you are hard pressed to find anything lesser than 5.1, even for re-released movies originally recorded in 2ch or less(!). But, more interestingly some movies shot even in 1968, like "The Planet of the Apes", "2001: A Space Odyssey" were recorded in 4 channel systems at that time according to imdb.