The audio buffer in FL Studio is a tempory data store of audio waiting to be sent to the soundcard and can be compared to bathtub full of water with the plug removed. If you pour water in at the same rate (or faster) than it’s flowing out, all is well. If the water level drops too low the drain will suck air and make gurgling noises. Similarly, if the audio buffer runs dry, FL Studio will make rude pop, click, glitch, stutter or static noises. It’s worth noting that underruns can ONLY occur in real-time playback, they will not happen while exporting to wave or mp3 file. There are many reasons why your CPU may fail to keep up with your soundcard, so this section outlines steps to ensure that your audio settings are optimized for FL Studio. Underrun minimization should be conducted with a typical project playing, your task will be to find the shortest buffer length setting with no (or very few) new underruns adding to the count. This process requires some trial and error as outlined below:

RECOMMENDATON: Select ASIO drivers if supported by your hardware.

ASIO Soundcard Drivers (ASIO will appear in the name)

The following is a summary from the Audio settings page. After each change, if the underrun count stops increasing, try to reduce the Buffer length setting further. Your goal is to find the shortest setting with no new underruns.

First, a reminder that as the Buffer length is increased, underruns decrease, but the delay between playing a MIDI keyboard, tweaking a knob and the response of FL Studio also increases. The aim is to minimize the buffer size without causing buffer underruns. For ASIO drivers, settings of 1-4 ms are 'cutting edge', 5-10 ms are 'excellent' and 11-20 ms are 'good'.
Make sure the Mixer Interpolation is set to Linear and the Sample rate is 48,000 Hz or less.
Increase the audio thread 'Priority' setting to 'Highest'.
Turn the 'Safe overloads' switch off.
Download the latest ASIO drivers from your soundcard manufacturer. We recommend sticking with the native ASIO drivers and only trying alternatives if you experience problems with them.
In some cases the 3rd party www.asio4all.com drivers outperform native ASIO drivers and may resolve underrun issues. Please be aware that you use this ASIO driver at your own risk.
Decrease polyphony of the instrument channels.
Turn off 'Keep on disk' for Sampler and Audio-Clip channels. This loads samples into memory which is faster.
Record mixer channels to audio and disable the instruments feeding those mixer channels.
Note: If your Buffer length setting is greater than 50 ms and your CPU usage meter peaks over 80%, it may be simply be your computer is not fast enough to play the project. Welcome to the never ending cycle of PC upgrades!

Standard Soundcard Drivers (If ASIO does not appear in the name)

First, a reminder that as the Buffer length is increased, underruns decrease, but the delay between playing a MIDI keyboard, tweaking a knob and the response of FL Studio also increases. The aim is to minimize the buffer size without causing buffer underruns. For standard drivers, settings of 5-10 ms are 'cutting edge', 11-20 ms are 'excellent' and 21-50 ms are 'good'.
Make sure the Mixer Interpolation is set to Linear and the Sample rate is 48000 Hz or less.
Increase the audio thread 'Priority' setting to 'Highest'.
Turn the 'Safe overloads' switch off.
Try each of the 4 possible combinations of the 'Use polling' and 'Use hardware buffer' switches. 4 combinations? Two switches with two states (on/off) mean 4 possible combinations, try them all.
Switch to ASIO mode (if supported by your soundcard). There is also 3rd party 'work-around' at a www.asio4all.com that allows many non native ASIO soundcards to operate in ASIO mode. Please be aware that you use this ASIO driver at your own risk.
Decrease polyphony of the instrument channels.
Turn off 'Keep on disk' for Sampler and Audio-Clip channels. This loads samples into memory which is faster.
Record mixer channels to audio and disable the instruments feeding those mixer channels.
Note: If your Buffer length setting is greater than 100 ms and your CPU usage meter peaks over 80%, it may be simply be your computer is not fast enough to play the project. Welcome to the never ending cycle of PC upgrades!