Software like FL Studio can be very demanding on your computer. When you start adding too many sounds to the project, the playback will start acting up, you will hear clicking sounds, or the software will freeze altogether.
It is suggested to have at least 2GB or RAM on your computer to run FL Studio. The default audio settings in FL Studio tend to work well when you have at least that much RAM. But as you start loading more sounds, or sometimes for no reason at all, the software will start acting up. This is manifested in clicks, pops, and crackling sound when you play audio. That's when you need to go to audio settings and tweak some controls, to remove the clicking sound hopefully.
And here's how you would go about it.
1). Go to Options Menu at the top
2). In the window that pops up, click the Audio tab on the left.
3). At the very top of the window, select a sound card driver. Try Primary Sound Driver first. If your computer starts to have trouble handling the load, switch to a better driver, such as ASIO4ALL.
Note: ASIO4ALL mutes sound in all other applications when you enable it in FL Studio. If you have some audio or video file playing online or on your computer when you switch to ASIO4ALL, your sound in FL Studio will not work. You will need to restart FL Studio and stop other sound playing processes, to activate sound in FL Studio.
Don't check the 'Auto close device' box. Otherwise it can stop playback of your FL Studio audio whenever another window from Windows pops up automatically. Very bad to thing to happen during live performances.
4). For Sample rate drop down, choose the sample rate which your computer could handle. 44100 Hz is usually the optimal setting for smooth playback. Higher sample rates produce better quality sound, but require more computer power. Lower settings lead to lower audio quality, but may be necessary if you have a slow computer, and would like to hear at least something. 44100 Hz is the MP3 CD quality, which, for all practical purposes, is good sound quality.
5). Move the Buffer length slider left or right, to remove clicks in audio playback if they occur. 20 ms - 40 ms is usually a good setting to start with. Longer buffer length means lighter load on the computer with stable audio performance, without the clipping sound. But if your computer has a lot of RAM, you can lower the buffer length, so that FL Studio can handle more. Most users would not notice any improvements in performance with settings lower than 10 ms.
6). If clicking persists, try toggling Use polling and Use hardware buffer on and off. 4 combinations are possible in all. Try different combinations of the two settings and see if that eliminates the clicking sound.