From YPPedia
Status commands
/away (/afk)

The /unmute command allows a player that has been muted to be heard again. That player will no longer have the "No" sign over their pirate's head and will be removed from the list of muted players stored in the client's options menu (players may also unmute pirates from here). If a pirate attempts to unmute a player that is not on the mute list, the game will print the message:
"X wasn't muted, nothing changed".

/unmute <pirate name>

Historical note

Before release 2005-06-01, the feedback message did not exist for unmuting a player that is not muted.