Alright, here's my report:
I did some testing, for any bugs, so I explored many combinations, so here are some videos, so you can see for yourselves:
1st one - ML600D - Audio - Bug 1:
Info: Here I was experimenting a little bit, I turned on the sound recorder, so only sound was recorded, then I pressed the record button for video and the camera went shut, it froze. At that point I couldn't do anything else but to remove the battery and restart the camera. I don't know if this is a bug or just a weird combination and a mixup of some wrong codes.
2nd one - ML600D - Audio - Bug 2:
Info: In this video I explored the possibility of recording secondary aka custom sound when recording video. You can see, that as soon as I turned on the sound-only recording, while I was video recording, I couldn't get out of the menu of magic lantern. Then I curiously pressed the shutter button, to take a picture, thinking if that would help, but it turned out, that I created the same freeze problem as in the 1st video.
3rd one - ML600D - Audio - Bug 3:
Info: Here I started only sound recording via the feature in the audo menu, and tried to change the functions in Canon's audio menu. When I did, Audio meters froze, and well...I'll let the video explain it, does a better job than me for sure.
4th one - ML600D - Audio - Bug 4:
Info: In the last video, I discovered that I was able to go from video mode to photo mode with the audio feature still turned on the whole time. So then I could record audio, even if I was in photo mode, because the audio was turned on from previous use of video mode.
So this could be very useful, to have audio also in photo mode and not in video mode, so that I could record longtime audio without having the shutter up and without the sensor getting hot through time. If this could be implemented, could you do it, miyake?
Oh and I also experienced the same bug from 3rd video, when I was making some changes with analog gain+boost and analog mic boost, while I was recording sound only. And I noticed that there was considerably less noise, if I was using analog mic boost to max instead using analog gain+boost to max.
Miyake I hope, this helps you in any way to make the audio function more stable for the 600D, but if not, I took some time out of your daily life and for that I am sorry!
Edit (28.08.2012, 17:21): I just found out another bug or false thing of some sorts. If, let's say, I wan't to turn off Override setting in Photo (M) mode and turning it on again, hoping that I could get audio in photo mode that way, my audio meters go crazy aka to the top of the metering range.