I experience significant sound lag (~0.8 s) in my Windows XP guest system when using VirtualBox (app-emulation/virtualbox-bin) 5.1.6.110634 and 5.1.8.111374. This problem does not exist in version 5.1.2.108956 (which now has been removed from portage).
Please report this to upstream (http://www.virtualbox.org) as we only use what upstream provides in the virtualbox-bin packages. Please note that the same issue happens with the non-bin package.