The "Audio" section in a virtual machine's Settings window determines whether the VM will see a sound card connected, and whether the audio output should be heard on the host system.
If audio is enabled for a guest, you can choose between the emulation of an Intel AC'97 controller, an Intel HD Audio controller[15] or a SoundBlaster 16 card. In any case, you can select what audio driver VirtualBox will use on the host.
On a Linux host, depending on your host configuration, you can also select between the OSS, ALSA or the PulseAudio subsystem. On newer Linux distributions, the PulseAudio subsystem should be preferred.
Since VirtualBox 5.0 only OSS is supported on Solaris hosts - the "Solaris Audio" audio backend is no longer supported on Solaris hosts.
[15] Intel HD Audio support was added with VirtualBox 4.0 because Windows 7 and later (as well as 64-bit Windows Vista) do not support the Intel AC'97 controller out of the box.