Only the Legacy VirtualBox paravirtualisation interface provides all of the BIOS interrupts and glue code that the Windows 98 installer requires. The Sound Blaster 16 audio card will work out of the box in Windows 98, but VirtualBox itself has trouble copying the virtual audio out onto the host, playing only 3 seconds of the audio at a time and skipping a little; eventually it crashes. So, use the ICH AC97 audio card instead, but then you need to find drivers for it.
As for the VGA adapter, that used to work out of the box (once you installed the VirtualBox Guest Additions), but the additions' support for Windows 98 was dropped a long time ago, so that's no longer an option either.
Put simply, you need the Legacy interface to run the installer, SciTech Display Doctor for the VGA card (which is now free), and Realtek AC97 Win9x audio drivers (which you can find very easily).
Then it all just works... for now.