Hi, Mark.
iTunes doesn't have a setting for mono playback.
Assuming you're running Windows XP, You may want to try going to the XP Control Panel > Sounds, Speech and Audio Devices > Change the speaker settings > Speaker Settings: Advanced button and - since it seems to be the only mono setting available - select Laptop mono speakers from the Speaker setup dropdown menu.
iTunes doesn't have a setting for mono playback.
Assuming you're running Windows XP, You may want to try going to the XP Control Panel > Sounds, Speech and Audio Devices > Change the speaker settings > Speaker Settings: Advanced button and - since it seems to be the only mono setting available - select Laptop mono speakers from the Speaker setup dropdown menu.