Sound is a menu in the options section of Super Smash Flash 2 that allows the player to adjust the volume levels of the game. It includes three options in three respective sliders the player can adjust at will: the general music, the sound effects, and the voices (including the announcer, playable and non-playable characters) of the game. Both the sounds for the menus and the sounds for the matches themselves get affected according to how the player wishes each option to be adjusted. The default level for all three sliders is at 70 but it can be risen up to 100 and lowering it to 0 will mute it altogether.
By coincidence, Super Smash Bros. for Wii U got almost exactly the same sound options arrangement as Super Smash Flash 2: both have three sliders for music, sound effects, and voices, where they are represented by red, blue and green respectively (though the last two colors are swapped in SSB Wii U) and the default volume level is 70.
It should be noted that the sound option was incorporated in version 0.6 of the SSF2 demo, released back in 2010, four years before Super Smash Bros. for Wii U was even released.
The opening movie is the only feature of the game that is not affected by the sound settings and will always play at the same volume.