emwhy
One thing I just noticed is that I have some older projects at 44.1 sample rate. I have Reaper set to 48, but if I open a 44.1 sample rate project it loads fine, but won't play until I change the setting in Reaper. Not a big deal, but maybe make some note in the documentation later on that this could be a minor issue. So far no issues with synths since the alpha was released. All tested MIDI and audio projects seem to loading well.
I indicate original sample rate, it should be in "Project Settings" / "Project sample rate".
For me, when a project with different sample rate is converted, my audio interface is switched to this rate and everything plays out of the box.
I guess in case your current interface does not support the original sample rate of the project, it won't play till you change in. Fortunately, unlike in Sonar, that is possible in Reaper.
For the moment, I do not see how I can improve the behavior on converter side.