1) video :
-play the demo in "time demo" mode (it will play all frames whatever time it'll take ; that's why you can do it on a low-end pc too)
note that there is a bug in 1.4.0 and 1.4.1 and you'll have to launch the timedemo once, wait until the game start (be no more in warmup), then disconnect and relaunch it, or else you'll not have a stable framerate.

-use the q4max cvar "demo_aviscreenshots" to capture every frame into screenshot (they will be stocked in "Quake 4/q4max/screenshots/avidemos/unknow")
here's a bind :
bind KP_ENTER "toggle demo_aviscreenshots"
you can choose to record in .jpg or .tga ; I recommand you .jpg because it takes less hdd space (a 15min demo takes something like +10GB .jpg, weight variations depending mostly on si_fps used). to choose .jpg use : "seta demo_aviscreenshotJPEG 1"
recording the screenshots in timedemo mod takes for my pc (average one) usually between 1 and 2 hours.

-once you got every screenshots, use virtualdub to mount them into an .avi :
open virtualdub, go in :
file and choose "open video file" ; choose then the 1st screenshot and it will open them all. this may take a while (5min for me)
video :
~in "filters" add the brightness filter (given below, you'll have to add it in "VirtualDub-1.7.1/plugins") and adjust gamma/contrast.
~in "framerate" :
a)"source rate adjustement" >> change it dependently from which si_fps the demo was recorded : for si_fps 90 you should enter a 90.909 framerate in virtuadub (see here for the other si_fps)
b)"frame rate conversion" >> change the framerate for what you think will be better in smoothness/video-weight ; I've chosen 40fps.
in "compression" >> choose the kind of compression ; for stage6 I had no choice but divx compression (I set it at "mode 1080HD, best quality, 4000kbps).
~be sure to have "full processing mode" checked.

then you are ready to encode (it generally takes 1 hour and a half for me) : File/save as avi

2)sound :
once you got the video, you need the sound :
use a software like Audacity to record the sound while playing the demo in normal mod ; do some testing before to set the right sound level in q4 (levels should hit around -6dB in audacity, without hitting into saturation at any time ofc)
then edit the sound to cut it right (the start of a match is easy to spot with the announcer voice yelling "fight"; I've done it with Soundforge cause I know how to use it, but I guess audacity can too.
record it to mp3 or else (for stage6 you'd better stick to well known encoding or it might crash the uploading process ; even if ogg is better, choose mp3)

-then mux the sound and the .avi : use virtualdubmod
in "file" > "open a video file" (open the .avi)
in "video" > "direct stream copy" (no need to touch the video)
in "streams" > stream list > add your sound file (direct stream copy too)
in "File" > save as
it takes 30 seconds

there you have your demo into an .avi, with sound.

upload it to stage6 ; once it's uploaded, if you got the "forever processing" bug, refresh once the page (works for me)
and you're done =)

I realise this is quite some work ; it takes time, and bugs & co slow down the process. hf =)