On older tranceivers the side-tone level was preset for you. On some of the recent 8.33 devices SIT or side-tone volume is pilot adjustable. To answer your question one needs to know the make and model of the tranceiver in the aeroplane and whether or not there is a separate intercom or you are using the one in the tranceiver.
If it is an 8.33 radio I would suggest a read of the manual.
If it is not.... Since you are in the UK you have until Monday to replace the tranceiver :-)