There are two separate problems with alac and the soundbridge. The first is that you must have a fast start enabled m4a file which some programs do not produce (e.g. dbpoweramp). You can run mp4creator -optimize -H=1 filename.m4a to add the needed info to a file.
An alac file which has fast start info will stream from iTunes->iTunes, from iTunes-Roku, and firefly->Roku via daap. It does not work firefly->Roku via rsp (at least for m4a’s produced by dbpoweramp+mp4creator, max, and omni encoder which uses iTunes to write alac. I have verifed this by running two separate instances of firefly svn-1498. One has the rsp plugins removed from the plugin directory forcing daap, the other has the daap plugins removed so rsp is used. They both read the same files on the server. The daap one streams alac to Roku just fine. The rsp version fails on each file with error message:
Write error: Broken pipe
The simple solution would be to just use daap however the combination of the current Roku beta and svn-1498 doesn’t browse very well – as an example browse artist gives you a list of every single album in the database with only those corresponding to the artist you selected having any songs once you go down the next level. i.e. rsp browses fine but doesn’t play the native alac and daap plays the files but has messed up browsing.
Hope this is clear – I’ve confused myself numerous times trying to isolate the alac problem. The URL I posted above links to a file that behaves this way.