One thing that is definitely true is that unless Sound Check is turned on, iTunes will not add iTunes_Norm information to files imported/added to the library from sources other than CD (ie, existing files dragged into the library)
Agreed, although I just tried importing some non-iTunes MP3's into iTunes with the Soundcheck feature turned on. As you say it went through the little Determine Volume routine but at the end of it after I checked these imported files tags in Foobar the iTUNNORM fields still had not been added and turning Soundcheck on and off during the playback of these files made no difference to volume whatsoever.
Interesting, so far I've found that
1. CD's imported (MP3 or AAC) get the Soundcheck info attached
2. Non-CD imported MP3's get the "Determine volume" scan (if Soundcheck on) but then no Soundcheck info gets added.
3. Non-CD Imported MP3's if converted to AAC in iTunes get the Soundcheck info added (with Soundcheck on or off). Same vice-versa AAC to MP3.
4. Non-CD imported (iTunes) AAC's get the "Determine volume" scan (if Soundcheck on) and Soundcheck info is added if not already present.
Sorry to be anal but I had to satisfy my own curiosity, looks like we were all right and wrong at the same time. It seems to depend on what your using and how you use it.
Going by that though, it seems that if you use EAC and LAME (as I do) you cannot apply Soundcheck to the files (and thus use it on the iPod) unless you convert them to AAC in iTunes. Can anyone confirm this ??
Edit: Okay, as far as I can see unless iTunes is used to import a CD to either AAC or MP3 then Soundcheck info will not be applied.
It works when importing AAC files created on another PC by iTunes, (but they should have the Soundcheck info there already).
MP3's created by other software (like EAC and LAME), when imported to iTunes, do not get the Soundcheck info added.
This might explain a lot of the "Soundcheck is rubbish, it doesn't work on my iPod" threads that appear here.
See below for further info.