The demos that can be launched from qtdemo.exe have their binary placed in a release subdirectory within the demo, e.g. in the mainwindow demo, the source lives in demos\mainwindow and the binary is demos\mainwindow\release\mainwindow.exe.

For the spectrum demo, there is no demos\spectrum\release subdirectory (the .exe is in demos\spectrum), so qtdemo fails to launch the demo even though it was built, and displays an error dialog suggesting that the binary wasn't built.

Moving the spectrum.exe to demos\spectrum\release allows qtdemo to launch it successfully, so the binary simply needs to be put in the right place during the build.

This problem seems to be due to the .pro file in the demo. Most of the projects under demos\... use a very similar .pro file, while the spectrum.pro is completely different.