Just a heads up - in case anyone else runs into this.
Just upgraded VirtualBox from 5.0.something to 5.1.8 and thought all was well. Until I tried to test a login sound - SOUND IS BROKE! (UM 16.04/1.14PPA host, U-M 16.04 and 16.11 guest).
Long story short, “unreliable DMA” on startup seems to be the core problem with default audio controller ICH AC97. Changing to Intel HD Audio works for me with some instability I notice but not bad.
I experienced this as well after upgrading. I found that changing the host audio driver from Pulseaudio to ALSA Audio Driver in the VM settings fixed the problem for me. Might be worth a try.