I've had a weird issue on a project I started before on X2 (or even X1?).
I tried to use "replace synth" because I since uninstalled the original VSTi. It doesn't matter which synth is used to replace - no sound is coming. I can create an entirely new instance of the synth and move the midi to that track and it works fine. If I use a synth with an on-screen keyboard like TruePianos No sound comes if I play it with the mouse. All the routing looks correct. I can split the simple track into an audio and midi track and the midi track's meter is displaying correctly when notes are hit. Its destination is correctly set to the new synth - that one just doesn't play. There are no plugins on the audio side and it's going straight to my main output.

Interestingly, and I just checked this, if I try to turn on the Prochannel EQ on the newly generated TruePianos audiotrack, it says I am using the Sonitus EQ (even though I'm very clearly not) and that I should control click to switch.

Any clues as to what might be going on?

Nobody?

Hi Sander,
 
After a synth replace you may need to actually turn the synth on, particularly if it is missing. Select the synth in the rack and from the drop down menu on the synth tab, choose Synth connected. Or, pop out the browser and turn on the power button.
 
Projects that contain the sonitus EQ as the inline EQ (from versions before X1) can be converted to the ProChannel Gloss EQ. CTRL click the power button to enable the Gloss EQ. This will not carry over any EQ settings from Sonitus so if you need those settings, continue to use that EQ by accessing it via double click on the plot on the Track Inspector or Console view. 
 

Thanks for your reply! That solved it. Excuse me for coming back with criticism but this seems incredibly counter-intuitive and convoluted to me. Why would you ever replace a "missing" synth with a disconnected one? Doesn't it make much more sense to make the default behavior a connected and active synth? Or at least give a pop up menu with that option. Is there a good reason for this that I'm missing?

I was aware of the Sonitus EQ thing, it just wasn't ever even activated on this particular track. Is that just a standard message for any and all tracks that were originally created without ProChannel? (This one was from X2 Studio I think)

I think it makes sense to be disconnected as the missing synth is an unknown, maybe not intuitive but easily learnt :)

In what situation would you want to replace a "missing" synth with a disconnected one? Genuinely interested.

all situations when a synth is missing

But wouldn't you want sound from it? Why replace it otherwise?

as a safety valve in case of a spike maybe

could be a replace option, auto reconnect