Have you tried to change the IRQ to 7? This would be done on the SET BLASTER line.
Also, as far as I recall, you do not need drivers for DOS, as most DOS games used direct hardware calls to drive the sound cards and other devices in the day. At least with QEMU you do not need drivers at all, just set the BLASTER line, even if that, and it just works.
For example, Ultima 6,7,8, and underworld all detect the sound card if the sound settings are set in the installer.
The only thing that would need drivers for DOS in VirtualBox would be the NIC cards, and yes it does work for file sharing with the Host.
Hope this information helps.