Set jabber-alert-muc-function to your desired
function. This function should look like:
(function nick group buffer text)
nick is the nickname, group is the JID of the group, buffer is the buffer where the message is displayed, and text is the text of the message.
The default function, jabber-muc-default-message, returns
“Message from nick in group” or “Message in
group”, the latter for messages from the room itself.
All MUC alert hooks take the same arguments plus the additional proposed-alert, which is the result of the specified message function.