Look at the screenshots. This is most likely an issue with PortAudio.

The method you're referring to is called additive synthesis. You may have better luck using the ADsynth engine.

LMMS uses the file projects/templates/default.mpt as the default template if it exists.

What a coincidence. I've written a similar tool with a very similar name in the same language :3

https://github.com/grejppi/lmmspackager

Feel free to examine it.