This video will quickly show you how to fix distorted / glitching / detuned Audio from a VST instrument inside FL Studio.
https://youtu.be/8RlElAJAkGs
If everything is crackling and glitching, then it's most likely due to buffer underrun (Your CPU is maxing out). In that case, check out my video on how to optimize CPU performance in FL Studio here:
https://www.youtube.com/watch?v=OWYmqwb7b4k
How To Fix Distorted/Glitching Audio From A VST Instrument In FL Studio (In 30 Seconds!)
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