Each audio file represents the pronunciation of a group of letters, each of which is a common set, minus one letter. The files are presented in alphabetical order:

1. AEHIKLMNOP[U]W: the letters of the Hawaiian alphabet
2. HBCN[O]FPSKVYIWU: one-letter chemical element symbols
3. KQRB[N]P: chess pieces
4. MDC[L]XVI: Roman numerals
5. OBAFG[K]M: star classification
6. ZX[C]VBNM: bottom row of letters on a standard keyboard

The six images at the bottom of the puzzle are each associated with one of these sets.

• Lei - Hawaiian alphabet - U
• Pawn - Chess pieces - N
• Colosseum - Roman numerals - L
• Beaker - Chemical elements - O
• Keyboard - Row of letters - C
• Star - Star classifications - K

The omitted letters, in picture order, spell the answer, UNLOCK.