WUPClient is pretty simple - all commands are just prepended with 'w.' - if you have experience in the command line you should be able to pick it up pretty quickly (I highly recommend not using the GUI - it is buggy/limited/sends the wrong download params)
So, like, `ls` (list files/directories) becomes `w.ls()` - I use fix94's version over on github (it can be found in the wupserver dir). The wupclient.py file is all the documentation I think you will need for commands.
`dl` is download a single file, `dldir` is download a directory and then `up` uploads file, etc. It isn't the fastest thing in the world but it gets the job done.
(just be really mindful to not mess with any system related files while in there - even though I did do regionhax while I was in mine - finally get to play one piece!)
The whole cfw build was pretty fast for me - but I already had my environment set up to do all the building. Be weary of using any pre-built cfw images. If you have any issues getting it set up PM me. I did it all on OSX but could still probably help/give pointers to building in other environments.
As far as audio size/length - that I really do not know. I have yet to play with audio tracks - it may be a matter of trial and error - sorry, I can't be of more help (yet at least - I plan on playing around with this over the coming weekend)