Why music software crackles and how to fix it
Crackles usually mean the buffer is too short. Raise it in Tools > Preferences in Loop by Zaweba; a longer buffer avoids dropouts.
Crackling in music software is nearly always the computer running out of time to prepare the sound, and the fix in Loop by Zaweba is to raise the buffer. Open Tools > Preferences (Ctrl+,) and choose a longer buffer, anywhere from 10 to 200 ms, with 30 ms as the default. A longer buffer gives the computer more breathing room and avoids dropouts; a shorter one feels more immediate when you play.
It's a balance, and the right setting depends on your computer and your song. Here's what's going on and how to find a setting that works.
What the buffer does
Loop works out the sound in small chunks and hands each chunk to Windows to play. The buffer is how much sound is prepared ahead. If the computer can't finish the next chunk before the current one runs out, there's a gap, and you hear it as a click, pop or crackle.
A bigger buffer means more sound waiting in the queue, so a brief hiccup doesn't cause a gap. The cost is delay: you hear things a little later after you press a key or a button.
Finding the right setting
Open Tools > Preferences. Check first that the sound output device is the one you are listening on. Then play the busiest part of your song, the bit with the most tracks and effects, because that is where crackling shows up first.
If you hear crackles, raise the buffer a step and play it again. Keep going until the song plays cleanly. If you're about to record with a MIDI keyboard or your computer keys and the delay feels sluggish, come back down a step at a time to the shortest setting that still plays cleanly.
Many people use two settings: a shorter buffer while recording parts, and a longer one while mixing a full song, when nothing is being played live.
Other things that help
Preferences also has a setting for rendering tracks in parallel, which spreads the work across your processor's cores. On most computers that helps a busy song play smoothly.
Close heavy programs. A video export or a game running in the background competes for the same processor.
Mind the memory. Loop uses about 0.6 GB with the General MIDI instruments loaded. On a computer short of memory, closing other apps helps too.
Lighten the song. Lots of effects on lots of tracks add up. Using the shared reverb and delay buses (bus A and bus B) instead of a separate reverb on every track is kinder to the processor, and often sounds better too.
Crackles that aren't the buffer
If the crackle is in the sound itself rather than in playback, the cause is different. A mix that is too loud distorts. Loop's master limiter has a ceiling of -0.3 dBFS, but tracks pushed hard into it, or into effects like distortion, can still sound rough. Bring the track faders down in the mixer.
A crackle that is always at the same place in a recording is part of the recording. Record that part again, or trim the bad bit.
Exports don't crackle
Exports render offline in the background, many times faster than real time, without needing to keep up with playback. So even if a heavy song crackles while playing on a slower computer, the exported WAV, MP3 or OGG will be clean. If you hear a crackle in an exported file too, it is in the mix, not the buffer.
If it still crackles
Right output chosen, buffer raised until it plays cleanly, parallel rendering on, other heavy programs closed. If you still hear crackles after that, check levels in the mixer before anything else.
