Start avast, then go to Settings, Sounds, and select 'Disable avast sounds'.
OR, if you just want to get rid of the siren when a virus is detected but wish to retain the other audible alerts (such as the 'new virus DB installed' notification), you can do this by clicking the button in that window, which opens the Sounds applet from the Control Panel. Then select the Sounds tab, scroll to 'avast antivirus' and change whatever you like...
Hope this helps,
Vlk