The first two options will produce an alert sound if any infected files are found and also whenever a scan completes. This allows you to go away, do something else and not pay too much attention to ClamXav while it's doing its thing.
Scan email content…
What this option does is allow ClamXav to scan the contents of individual email messages for malware. This option frequently flags up emails which other malware scanners may miss, however, if you use something other than Apple's supplied Mail.app (eg Outlook, Entourage, Postbox, Thunderbird etc) then you should keep this option turned OFF.
Turning this option on does not mean that ClamXav will automatically scan your email - you still need to tell ClamXav to scan the appropriate folder containing your email.
If you use something other than the Apple supplied Mail app (eg Thunderbird, Postbox, Outlook, Entourage) we recommend leaving this option turned off for now.
Update virus definitions on launch
Virus definitions are small snippets of code which enable ClamXav to detect malware and must be kept up-to-date for ClamXav to be effective. These definitions are updated very frequently, sometimes as often as twice a day. This option should be enabled to obtain these updated code snippets every time you use ClamXav.
Check for ClamXav update on launch
This option allows ClamXav to ensure the application itself is always up-to-date. When an update is available, all you need to do is hit the "Install Update" button.