When it comes to a flexible audio setup Alsa starts to suck ... hard. If you want output mainly on S/PDIF (be it optical or coax), automatically upmixed to 5.1, then encoded to A52 so your receiver will eat it, but sometimes your general AC3-passthrough 5.1 movie, sometimes maybe even output to your regular analog output ... then its quickly becoming clear that ALSA is basically a driver framework, and not a end-user audio application. I found that most audio middlewares i've known sucked quite hard: Jack is too unuseable, arts or phonon never did their job, esound is kind of ... dead. So what's left? PulseAudio is left! Its setup seems very quirky to firsttime users and if you come with some strange distro-configuration (*cough*ubuntu*cough*) you may want to throw it against your wall ... but actually its super flexible and nice.
If you have a similar setup to mine - read on!