Personally, I feel that instrumental interludes are excellent opportunities for inserting original voice and sfx clips into the song although you can't just insert them at random with impunity. Probably one of the main reasons my first video had a good overall 'feel' is because right at the climatic guitar build-up instrumental interlude I inserted the sounds from the climatic arm slave clash from the anime. It just seemed to fit so well and the fact that that segment matched so well to the instrumental interlude was one of the main reasons I wanted to use Luna Sea's Shine with fullmetal.panic!
In Japanese music OSTs, this is actually a pretty common occurrence. If you have listened to the Noir CDs, there's a dramatic version of salvanos with audio clips from the anime.
I'm also personally planning on doing an AMV instrumental, but then inserting a whole bunch of audio and sfx into it for dramatic effect in the future.
If you've seen the Professional GGXPrev anime 'trailer' you can get a good idea of what I'm getting at. It uses three songs from the GGX OST, which are all instrumentals but there's so much emotive talking and appropriate sfx that matches the songs so well it makes the words and actions somehow more impactive than if they were sung. It's an interesting concept but only Japanese artists have utilized it before as far as I know, what with a good portion of their game and anime CDs being instrumentals.