Ceiling. Definitely the ceiling...and a pile of carriage bolts.
or floor - I currently have 3 cases on my living room floor and a video monitor on the coffee table!!!
More seriously, if you're going to have an audio input, you'll also want it to output envelope signals. Have a look at Doepfer's A-119, which is pretty much the standard for these. Not only do you get the necessary preamp, you also get an envelope follower AND a comparator. The comparator sends a gate when the input level exceeds a certain threshold, and the envelope follower does pretty much what it sounds like: it extracts level information from the inputted signal and then converts the amplitude variations as a control voltage...which, if you run the audio signal through a VCF and then use the envelope follower to control the VCF's cutoff, giving you your original sound but with this synced-up filter sweep imposed on it.
-- Lugia
whilst I completely agree with the need for both envelope follower and comparator for external inputs - it might be useful to point out that there's a 1/2 decent envelope/pich follower algo in disting - and kinks will do the comparator - so if they're not needed all the time - maybe these will suffice...
"some of the best base-level info to remember can be found in Jim's sigfile" @Lugia
Utility modules are the dull polish that makes the shiny modules actually shine!!!
sound sources < sound modifiers < modulation sources < utilities