You need to add each soundfont that you want to be able to select for track instrument under the Soundfonts tab. The “Scan Soundfonts…” button will add all soundfonts in the selected directory and sub-directories but beware: it may not be a good idea to add hundreds or thousands of soundfonts, instead use a small set of high quality soundfonts for your compositions.
It is important to set the Soundfont Gain properly. The sound should not clip or be too quiet.
It is recommended to enable the Reverb and Chorus which will create more natural, not too dry sound.
Soundfonts are automatically added/removed from the 'Soundfonts' directory in
Windows: C:\Users\<username>\Documents\RapidComposerV3\Soundfonts
OS X: ~/Documents/RapidComposerV3/Soundfonts
If you copy an .sf2 file into that directory, it will be available in RapidComposer the next time it runs.