How long is the sound? It's often not the length though, AFAIK, it could just have too many channels in it as well. I'd always had trouble converting MIDIs to AGI or SCI, because the MIDI format is a little like the AVI format, in that just because they're AVI, doesn't mean they are normal AVI--the could be divx, xvid, 3-vix, mpeg-2, dvd-mpeg, etc. Many MIDIs are varied formats, and can't be converted simply.
I think you need to get a midi editor and strip it down and/or convert it to standard midi.
Ravi created the Sound utils, and Lars knows everything about SCI right down to the smallest detail, so they both should be able to answer your sound questions.