I first did this mod so I could use the Monotribe as a filter. The stock Monotribe mixes in its own synth voice with the audio input, and there's no normal way to defeat it. When you turn off the synth voice on the sequencer, it mutes the Audio In as well, kind of killing the fun of the idea of using the 'tribe as a filter. In comes the soldering iron... When you mute the oscillators, the synth voice is muted, so you can run whatever audio you like into the Audio In jack and still go through the filter, and the LFO still works too. Nice. The catch is I think you have to have the synth voice running a pattern, even though you can't hear it, or else the whole thing gets muted. Not a problem really just turn the unit on and it's got that stupid pattern in it. Which brings me to the only issue I've found with my mod: You have to turn the Monotribe ON with the VCA not muted. If you have the VCA switch set to mute (ground), the Monotribe will hang on a blinking light and will not boot up. Just turn it off when you're turning it on ;-D.
But I found that even when not using the Audio In, when you mute the VCA, you can set the filter resonance really high and just "play the filter!" which is...so cool. It opens up even more room for high pitched, weirdo, alien, video game, serial killer soul stealing type noise making.
So yes, between the MIDI mod and this one, I'd recommend doing both for more fun and features.
My switch location was not optimal...I had to hack at the case a bit, the bottom half, to get it back together again. So as always, drill carefully and with a plan, if you can.
I've been told this mod can be done effectively with an SPST switch also, if you don't have a 3PDT or don't want to use one. That would save some space inside.
UPDATE (6/5/2013) -- It turns out the filter does key tracking, so, if you program in some kind of melody on the ribbon, you get a tempo synced step filter! Pretty cool, I just wrote a song with this, using my DX7 into the Monotribe filter section. It also turns out that the envelope generator is still active, so, your "invisible" melody will fade up, or saw down, or do nothing (square) depending on which envelope you select, and how many separate notes you play in. You can also sort of "play" the cutoff in a small range on the ribbon instead of the usual cutoff knob. If you want a static filter with no stepping or sweeping, you can just hold down a single note for the duration of the monotribe's sequence length, and set the EG to square.
I'm not disappointed in the filter, but, it doesn't sound like the MS10 filter demonstrated here http://www.bebetterstudios.com/blog/2013/03/04/monotribe-midi-modification/. At lower ranges it seems to get "quiet" instead of "fat." It's also kind of a noisy situation, even compared to the DX7 hiss. So yeah that's a little disappointing overall, but, the syncable step filter and LFO filter, envelope sequencing makes up for this, for sure. This is a lot of fun, you've just got to try it!!
This $5 computer software http://www.fabriziopoce.com/MTribe.html for your MIDI enabled Monotribe, looks to allow adjustment of key sync, and many other things "under the hood." I'll be trying it out.