When FastChords starts, sound is turned on.  You can tell when sound is ON because the sound buttons look like this:

This is Sound ON/OFF button.  Clicking on it alternates between sound ON and sound OFF.  When sound is OFF the button looks like this::

The crossed red circle on top of the speaker icon indicates sound is OFF.

When sound is ON, clicking on the title bar of the keyboard plays all the keys that are down.  These are the selected keys.  To select / unselect a key use the right mouse button.  See the nest section "Playing with the Keyboard".  Keys can also be selected from the musical instrument using midi IN input.  See Selecting a Midi IN Device.

Clicking on any individual key sounds the corresponding piano key.

FastChords uses the Direct Sound component of Microsoft DirectX version 8.0 and above.   The program will

check whether you have the proper version of Microsoft DirectX and helps you install it if necessary.

   DirectX          midi

Anywhere in the program, whether on chords, table, melody or play modes, the background color of the speaker button tells whether DirectX sound or midi sound is being produced.  See Midi and DirectX sound.