@Hubbit200 first of all conversion to MP3 doesn’t actually reduce the memory usage, all audio samples are ultimately stored uncompressed in RAM, so as you’ve noticed all it does is increase the time to load the file.
Regarding clearing sounds from RAM, like in any Java program this is done through automatic garbage collection, so in theory all you need to do is make sure that there are no more references to the loaded audio file in your program, i.e. something like:
file = new SoundFile(this, "sample.mp3");
file.play();
...
file.stop();
file = null;
(I have a hunch that any loaded files might remain in memory anyway because of how the Sound library caches them, but if more people are concerned about being able to clear old sound files from RAM we could definitely add an argument/method for doing so!)