Why Ableton Can't Help

Sidechain compression needs the trigger signal to actually reach the compressor doing the ducking. Ableton's stock Compressor can only target something already inside Live, a track or a specific chain, through its Sidechain > Audio From dropdown. If the Electribe's output never gets routed back into the computer, there's nothing in that dropdown to point at.

The usual workaround when a drum sound lives outside the DAW is to fake the trigger: duplicate the MIDI notes driving the external drums onto a dummy track loaded with a simple instrument, route that dummy track's output to sends-only so it's inaudible, and sidechain off that instead of the real thing. That trick depends on the drums being MIDI-triggered from inside Ableton in the first place, so there's a MIDI clip to duplicate. Once the drums moved to the Electribe's own internal sequencer, that MIDI clip stopped existing. No trigger to fake, nothing to duplicate.

The Real Answer: Do It After Tracking

Real sidechain ducking across an all-hardware signal path needs a compressor with an actual external key input jack, patched so the target instrument runs through it and the drum signal feeds the key input separately. That's a real, fairly cheap category of gear, but not something currently in the rig.

The workaround that doesn't require buying anything: stop trying to do it live, do it after the parts are tracked. Once the Electribe's drums and the other instrument are recorded to separate audio tracks in Reaper, whatever generated them stops mattering, they're just two audio signals sitting in the same DAW. At that point:

1. Add ReaComp on the FX chain of the track that should get ducked.
2. On the drum track, add a send to the target track, routed to channels 3/4 instead of the normal 1/2.
3. In ReaComp's sidechain settings on the target track, set the detector input to channels 3/4.
4. Don't route that send to the target track's fader or main output, it only feeds the compressor's detector, it's not supposed to be audible on its own.

Same Threshold/Ratio/Attack/Release tuning as any other sidechain setup, the only difference is Reaper's channel-based routing instead of a simple track-name dropdown. This only works if the two sources were recorded to separate tracks in the first place, not already mixed down together, so that part has to be planned before tracking starts, not fixed afterward.