Ah, yes that's why in DC, although most (all now I think) DCC silent or sound equipped locos will run on DC from the factory, the silent ones (whether they have a DCC decoder or not) will do that. As I mentioned it takes 5-7 volts, depending on brand, to get just the sound going, and if the engine's on a DC track, then the silent ones are already rolling (or trying to), whereas the sound equipped won't normally move until the sound has gone through the motor start-up sequence, consequently dragging it's feet. As you've noticed, dead spots have the same effect. So, not really possible to consist on DC.
Using DCC it's a different matter (just don't ever put a DC only loco on a DCC track), there you can consist both DCC/sound and DCC silent decodered engines. Generally speaking most sound locos will go into motor sound start-up on contact with a live track, or if on the track, as soon as the power is switched on. The silent decoders also, even though they're silent, are "fired up" and ready to roll. Some sound brands require the pressing of a function button to start the sound on, but they all recommend waiting till the motor has settled back to idle, before moving off. The actual point of starting to move, because power is at maximum on the rails all the time, is determined in the settings of each locos decoder, so it will apply electricity to the motor at (normally), speed step 1 on your cab's display. This is the realm of speed matching.
