In both cases, you want to edit the Tags on your songs. Album Artist and Album Artist (auto) are both tags. To change tags, choose some files by highlighting them. Then right click and select "Tag". The Tagging Pane will open on the left side. Now you can change the tags on all of those files at once. To see all of the tags, click the upper left corner of the Tagging Pane to get a menu. From there you can select to see all tags, or just some of them.
1. For your multiple artist album, change the Album Artist tag to something like "(Multiple Artists)", or "Various" (without the quotes). Don't use Album Artist (auto). Use Album Artist. The system will automatically fill out Album Artist (auto) for you.
2. For the album that has several discs you want to set several tags for each disc:
A. Album. Right now, every disc in the album probably has as different name ending with (Disc 1) or something like that. Change the Album tag of every file in every disc to be the same. You can do this all in one operation by highlighting all of the files and then changing the Album tag for all of them at one time. But don't do this yet!! Keep reading.
B. Disc # . Set this to be the correct disc number for all the files in each disc. So set it to 1 for Disc 1, 2 for Disc 2, etc.
C. Total Discs. Set this to be the total number of discs in the Album. Not strictly necessary, but you want to do this for completeness.
After you've set the Disc # for all of them, *Then* go back and set the Album to be the same for all of them. That way, you can still tell which songs are on which discs, as you set the Disc # field. If you did it in reverse, you'd lose track of which songs are on which discs.
Good luck, and let us know how you do.
Brian.