DS918+ as a host has no audio hardware which would otherwise be accesible by VMM (or KVM to be precise) hypervisor and further available to VMs. In order to get some audio on VM, you use your local PC audio hardware as a playback renderer. You should create a custom RDP connection, set some settings and save it as an icon for ease of use. When invoking Remote Desktop Connection, under 'Show options' select 'Local resources', then click 'Settings' under Remote Audio section. You should see 'Play on this computer' checked under Remote audio playback section. This should be enough for audio.