there's a similar thread around here somewhere, a few months old iirc...
There are problems with your search string, most notably, the ~a modifier.
What does MC actually do with your string?
[File Type]=[mp3] - lists all the mp3's in your library using the prevailing sort instructions
~s=700 - limits this list to 700 mb
~a - tells MC to take each file in this 700 mb list and add all other files in your library that match the [album] tag (which in turn breaks the 700 mb limit specified earlier)
completealbum==1 - from this now mega huge list, discard all files that are not flagged as complete albums.
You might find that simply removing the ~a modifier will be enough for your needs, though I'd also advise moving the ~S=700 instruction to the end of the string.
What is not clear to me, is whether you want the random nature of your mp3's to be by file, or by album?
ie. do you want them to play one random album after another, or do you want them to play tracks from all the albums in a completely random order, but also want to have every track from the albums included in there somewhere?
this string:
[file type]=mp3 completealbum==1 [track #]==1 ~sort=random ~a ~s=700
pulls me 700 mb worth of random, complete albums. except for the last album in the list, which is always cut short before it breaks the set size limit. if you want them all jumbled up, add another ~sort=random to the end of the string.
that any good for you?
-marko.