Why can I play a complicated song or make on-the-fly changes without the sound getting distorted and breaking up?
This is almost always a latency issue. Select Edit > Preferences… from the menu. Then go to the audio page and increase your Output Latency. Reason’s default settings usually aren’t the best in this area, so adjust according to your computer and soundcard settings. Play a busy part of one of the demo songs and change some parameters while it’s playing to test the settings. Increase your output latency until you hear no irregular pops or cracks.
