Quoted from cpr9999:Assuming your left/right backbox speakers are working.
If so, open backbox and look in the lower left corner for the audio board. At the bottom of the board is where the speakers plug into the board. Check them.
If all connected, lift playfield and check the connectors on the speaker are connected.
If all connectors are on:
Then check for either wired backwards OR cracked wire between big speaker and audio board.
If all above is correct: hook speaker up to another game to check.
If good, then bad audio board.
This all assumes there is no setting to turn off big speaker in code menu.