GBPVR uses Microsoft DirectShow (DirectX component) for playback. Normally sound devices don't provide a Directshow (software) decoder, but do provide a DirectShow output interface (audio renderer) which the DirectShow decoder outputs into. Some sound devices also have inbuilt hardware decoder which you can use, however the DirectShow signal path still requires a software decoder. In the case of using a hardware decoder you configure the software decoder for SPDIF pass-through, which simply forwards the raw encoded audio to the sound device for processing. In summary, I would be surprised if you sound device includes a DirectShow decoder, in which case you will need to install a software decoder anyway.
The Purevideo Bronze product is crippled to 2-channel only. This includes SPDIF pass-through function. Many people have been tricked into thinking that SPDIF still forwards all channels, but it is actually down-mixed to 2-channel PCM with dolby pro-logic within the SPDIF signal, and people's soundcard/receivers convert it back to surround-sound or in many cases the soundcard/receiver simulation circuitry emulates the surround-sound. Creative soundcards are amoung the worst for tricking you because their simulation function (CMSS) is enabled by default. The Purevideo Bronze product is fine if you are not playing any surround-sound material.
The Purevideo Gold product has 2-channel and Dolby Digital, but not DTS on SPDIF pass-through or software decoding.
The Purevideo Platinum product has 2-channel, Dolby Digital, and DTS on SPDIF pass-through or software decoding.
You don't have to use both the Purevideo video and audio decoders together, but in my experience if you use a different audio decoder you may experience weird side-effects like in my case the DVD FF/RW buttons stopped working when I used a different audio decoder.
AMD Athlon X2 4200+ CPU, Gigabyte GA-MA770-DS3 mobo, 2GB RAM, 1TB SATA HDD, DigitalNow Dual Hybrid PCIE S2 and Hauppauge HVR2200 capture, ATI HD4670 video with HDMI-HDMI to 32" LCD TV at 1360x768, Win7 Home Premium 64bit, GBPVR 1.4.7, EVR renderer