Petes,
Audio Bitstreaming is player dependent. i.e you have to enable bitstreming in you video player.
If you are using xbmc (eden) player, in its audio settings tab you have option to choose speaker settings (number of analogue channels) as well as audio formats you need to bitstream.
If you are using KLM Codec pack with media player classic home cinema, while playing a movie file, right click on screen -> Filters -> there you should see the audio decoder filter (might be ffdshow audio decoder, or LAV audio decoder - depending on the version of KLM codec pack / audio decoder packs you installed). If its ffdshow audio decoder, go to its properties window and then to output tab. There you have option to Passthrough (SPDIF/HDMI) audio. Choose the audio formats you would like to pass through and that is supported by your graphics card/receiver combination (DTS, Dolby Digital, DTS HD, TrueHD, etc).
Also for this to work, your player as well as audio codec you use, must be registered as the primary codec for decoding that audio format. i.e in the above KLM codec pack example, if somehow while playing a video file with DTS audio, the player loads LAV audio decoder for decoding audio instead if ffdshow audio decoder; there wil not be any bitstreaming as you have not configured bitstreaming in LAV audio decoder(or any such audio decoder).
Hope this helps. If you use another player please google bitstreaming + your palyer name
XBMC is pretty much straight forward.