A channel audit is an occasional run through of all of the channels you've joined and deciding whether you still need to be in them. In general, if you haven't contributed to a channel and/or have had it muted for >1 month, you might want to consider leaving.
To leave a channel, simply type
/leave in the message box.
If you come across a channel that you own that has gone dormant (meaning that no one has been posting for >1 month), please archive the channel instead of simply leaving it.
To archive a channel, simply type
/archive in the message box.
Don't worry! You can always reactivate the channel.