I guess my initial question should be worded differently. Basically, I see a number of reasons for something to be included in the MIC:
1. It was in another book (DMG or other), but something needed correcting/updating (price, errata, whatever).
2. It was in a book other than the DMG, nothing needed to be corrected, and it was good/cool/best enough to be included in the MIC, even if it may have been a campaign-specific (e.g. Eberron) or optional-rules-specific (e.g. Magic of Incarnum) item.
as well as a number of reasons for something NOT to be included in the MIC:
1. It was in the DMG and needed no correcting/updating.
2. It was in a book other than the DMG, nothing needed to be corrected, it was NOT good/cool/best enough to be included in the MIC, but the item itself is still a sound item to use.
3. The item was fine enough in its original source book, but something about it made it too campaign-specific (e.g. Eberron) or optional-rules-specific (e.g. Magic of Incarnum) to be included in the MIC, which is presumably a book of items usable in most any D&D campaign.
4. It was in another book (DMG or other), but it was SO bad that it was decided that scrapping it from the game entirely was better than trying to correct/update it to be included in the MIC.
So, when I (mistakenly) thought that sonic burst (now screaming burst) was not in the MIC, I had no way of knowing which of the above reasons (or another I didn't think of) was WHY it was not included.
Even though my original example is moot since it IS in the MIC as screaming burst, I think the question is relevant for any item not found in either the DMG or the MIC.
Thanks,
Atavar