Your buffer size may be too small, resulting in buffer underruns. To fix this, increase your buffer size. Note that this will increase latency.
Certain audio interface configurations don't play nice with LMMS on some systems. If you are on macOS or Windows, make sure you have SDL selected. On Linux, if PulseAudio is producing these unwanted artifacts, try ALSA instead.
Your audio within LMMS may be too loud, resulting in digital clipping. This can be fixed by reducing the volume of the instruments in LMMS (either with the "VOL" knobs or through the mixer).
If an instrument has attack or release values small enough, the rapid change in volume can sound like clicks or pops at the beginning or end of each note. Increasing the attack and release values for the culprit instrument's volume envelope can fix this kind of clicking.
Certain audio interface configurations (specifically Dummy) just don't work sometimes. If you are on macOS or Windows, make sure you have SDL selected. Try to avoid PortAudio if possible.
If you have JACK selected as your audio interface and you are experiencing no audio output, JACK-transport might not be running. Start an application like
qjackctl and click on "play" or its equivalent. LMMS should automatically should start JACK-transport on startup, but this fails on some systems.
You may also need to connect LMMS to your sound card, using the patch bay in
Buffer size is directly related to latency. If you can, try decreasing your buffer size to minimize latency.
Use the GNU Debugger to run LMMS again, and try to replicate the crash:
$ gdb lmmsGNU gdb 6.XCopyright (C) 2006 Free Software Foundation, Inc....(gdb) rStarting program: lmms...
If a segfault happens, LMMS will freeze and you'll get some information on the terminal. Entering
bt full will show a backtrace you can report.
Make sure you have debugging symbols in LMMS. That usually means compiling with the
-g flag and not stripping the symbols afterwards.
Make sure you didn't pass
-fomit-frame-pointer in a FLAG-parameter to configure. This is known to lead to an unusable LVSL (LMMS VST Support Layer).