It is the ACELP.NET codec which seems to be a common denominator across all Microsoft Media 6 , 7, and 8 installations. I tried re-installing but it doesn't show up in Vegas 5 just in Vegas 4.

Hmm....I'm sure there is a good reason, but the 'why' escapes me. You do a lot of encoding of voice for streaming?
It might be that VoiceAge has changed something relevant to install...as reinstalling should bring it back to Vegas unless something is switched. I'd recommend contacting VoiceAge in the meantime. Sorry I can't be more help. ACELP.NET codec is a good one, recommended by REAL and Microsoft both for voice work. We've only had a couple requests in the distant past for it, so not a lot of familiarity with it.

I'm constantly concerned with backwards compatibility of WMV streams, as I have a client with a large installed base of WinNT4.x machines (which topped out with WMP6.4)

So ACELP.NET is a pretty important audio codec! However, you should be aware that although WMV9 VIDEO codecs are not backwards compatible to WMP6.4, WMV9 AUDIO codecs are! Of course the client somehow needs to get the codec, and AFAIK robust automatic download starts with WMP7.x.

FWIW.

Our issue with the WM9 codecs relates to the fact that we produce product that is targetted at large organizations including government departments where users do not have administrative rights on their PC and hence the missing codec downloaded in the background doesn't get registered and hence doesn't work. I believe the ACELP.net codec was included with all versions of WM prior to version 9.

WMV9 VIDEO codecs are backwards compatible to WMP6.4 if you install the wmv9 video only codec for it .... not the player ..

i use 6,4 on a older sgi running nt4 just fine