After a fresh install, or booting a live cd/dvd under virtualbox, it's necessary to run pavucontrol, and change the output device from Analog Mono Output / Amplifier to LFE on Separate Mono Output / Amplifier before sound is working. This is using the default vb options, with pulseaudio on the host.
duplicate of bug 5509?
*** This bug has been marked as a duplicate of bug 5509 ***
Status: NEW => RESOLVEDResolution: (none) => DUPLICATE