Open LMMS and load your project
Import the audio file into an Audio File Processor track
Select the Audio File Processor track
Open the track’s settings or instrument window
Locate the volume or gain control
Automate the volume using an automation track
Draw a fade-in curve at the start of the audio clip
Adjust the fade length by moving the automation points
Alternatively, use the clip’s volume envelope if available
Save the project when the fade sounds correct