The AudioFileProcessor plugin is used to play back uncompressed or compressed samples in an instrument. It looks like this:
The amplitude for playback of the wave file, in percentage of the original. You can go above 100% if your wave file is quiet and you want to make it louder. If the master volume or the volume of the instrument or track is low enough you won't cause any clipping, because of the way the audio signal is processed in LMMS, naturally, you can also quiet a sound down by using percentages less than 100%. However, this won't correct clipping that was present in the original file.
The start and end of playback of the sample can be used if you only want to play back a specific part of a sample.
The reversed R controls the direction of playback. When inactive, the sample is played forward; when active, the sample is played in reverse.
The cyclic arrows control whether the sample is repeated from the beginning once it reaches the end (or vice versa for reversed playback) if the note is still held down and playback has progressed all the way through the file.
The icon with two bars and a marker enables stutter-mode. In this mode, sample playback is always started from the point where it last ended. You can reset the position back to the beginning by playing a very low note, one that is below 20hz in frequency.
The folder icon allows you to choose a new file from a file open dialog. You can also change the current sample by dragging a sample from the Samples section of the Side Bar.