Now if you are using an AVR Receiver / System that connects to your Media Center via HDMI, this whole post is a don't care. But for those out there that have surround sound setups whereas the only connection between Media Center PC and their surround system is Spdif then you do care about this post.
After much research, I have determine how to get Dolby Digital 5.1 from both Amazon Instant & Netflix. But first here is some background information.
1) All streaming devices; Roku, etc. encodes the audio using Dolby Digital Live. So basically they convert the decompressed LPCM/PCM audio streams to Dolby Digital 5.1.
2) Amazon Instant Video Unbox downloads and streaming videos uses WMA Pro audio. If you read on in Amazon's help section they do say that they send out 5.1 audio, but they do-not comment on the encoding methods.
3) Netflix uses Silver-light and they are sending out multiple channels of PCM audio.
4) As for PlayOn, its stereo only.
5) Just because they say it 5.1 doesn't mean that it's Dolby Digital 5.1.
6) I have not tested Hulu yet, but I will later.
So what is needed is an AC 3 encoder (aka. AC 3 Filter) to encode the PCM/LPCM audio streams to Dolby Digital 5.1. But AC 3 Filter does not work on Amazon/Netflix audio stream because of CONTENT PROTECTION(DRM). So the alternative is Dolby Digital Live. That right Dolby Digital Live is a DRM enabled AC 3 Encoder.
A few years ago, motherboards manufacturers would include the Dolby Digital Live encode on their software CD. In order to include the software they had to pay a license fee to Dolby for every motherboard sold. To cut their cost most of them decided not to pay the license fee. But they still advertise that their mother board supports Dolby PCEE and don't include the software. Dolby PCEE software includes Dolby Digital Live.
I recently built a new Medial Center PC (details 21st century) and used an AsRock motherboard. The motherboard specifications included Realtek 892 audio codec with Content Protection and Dolby PCEE. There was no Dolby PCEE software on the included CD.
So in order to get Dolby Digital 5.1 for both Amazon and Netflix I purchase this;
The software pack for the Creative Sound Blaster X-Fi Surround 5.1 Pro USB Sound Card included the Dolby Digital Live encoder. There are a few hiccups with this setup, but I have to work on them.
I have now fixed a lot of the hiccups that I was having.
So we should all complain to the motherboards manufacturers for not including the software.