I have no idea how LVM improves or not Vbox performance. Under Xen it is the recommended way. LVM has its own advantages, as you can grow virtual disks to use several partitions or real disks as if they were one disk/partition, or even stripe two disks to get RAID0 performance.
I doubt LVM will improve audio performance! In fact, even under Xen you'll have to pass-through the PCI ID for the audio device to get native performance. I haven't used audio yet on my Windows 7 guest, since I only passed through the audio device on the graphics card and I don't have an HDMI link to make use of it (use DVI). If your screen has HDMI and built-in speakers this should work well, provided that you use Xen PCI passthrough and have the required hardware (this is the trickiest part with Xen PCI passthrough, as not many video cards are supported - mainly AMD - and you must have VT-d / IOMMU support in your CPU and motherboard).