Ceiling. Definitely the ceiling...and a pile of carriage bolts.
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.