Noatikl doesn't include any sound generators; Noatikl is a pure MIDI generation engine. That is to say, Noatikl generates MIDI events that are used to trigger sounds that are created by other software in your system.
On Windows, Noatikl will by default play sounds through the Microsoft software synthesizer that is built-in to Windows. On Mac, you have to do a little more work to play sounds through the software synthesizer that is built-in to Mac. But in general, once you've got Noatikl up-and-running, you'll want to play Noatikl through your very best synthesizers!
So, in order to use your own sounds, you'll need to send the output from Mixtikl into your favourite synthesizers.
The actual way you do this, differs very much depending on what desktop audio tool you use (e.g. Logic, Reaper etc.), and whether you're using Windows or Mac....
The note durations output by Noatikl are practically unlimited (in the case of Ambient voices). How good very long notes would sound, depends on your target synth.