My memory of Windows XP has faded these days, and have zero to bog all knowledge of Windows 7, but since nobody has any better ideas, ill chime in with a couple of possibilities.
Are drivers for the virtualbox NIC installed in windows 7?
If so, can the Win7 guest ping the host?
Can it ping an external IP (google, 22.214.171.124)? If so, this would suggest that it is a nameserver problem, which you can probably fix by manually setting the name servers on the Win7 guest, to googles dns servers.
I assume you haven't diddled about with the network settings of the VM itself, and that is all set to default. If not, it may be worth testing it with a new vm using all the default settings.
As for your sound issue, i have no idea. But testing it with an alternate media player may diagnose whether it is a vlc problem, or a general sound problem.