maybe a page in firefox opened a second site with a video chat room?
i have seen many of those in opera, firefox and chrome recently, the browser can't stop them anymore from opening without your authorization
also could be a video she opened but didn't loaded so she left it unattended and when it finally could be played made the noise?
that could explain the sound but the logs you mention are completely different, not sure if related or not and what that means, if a real intrusion or just that the system deleted old entries
the machine has a firewall if i understood correctly but sometimes a router is a better option than connect directly to the modem
i'm not sure if you could put one for a extra layer of protection without making things more complicated
it will have wifi but if you don't use it or don't want it, some d-link models will let you disable the wifi signal