Instructions
Log out of the PSN and start the game that you want to play. Press the "Cycle" button and you will see a prompt to play the game in "Offline" line mode. Select "Yes" to start the game in Offline Mode.
Go to the "Main Menu" for the game console and select "Options."
Find the option for "Sound Settings" and select the option for "Headset Volume." Set the sound level for the headset to "zero" to mute all of the players for the game.
Log back onto PSN and restart the game and all of the game players will be muted when a headset or a pair of speakers is used.