Greenlight? I had to do a double take here to realise that you weren't the original poster. Meaning, you are asking something different to speeding-up video. Maybe a separate post would help?
Anyways, we ARE here now, and maybe I can attempt to address your issue.
"
The problem I have with the velocity envelope is that it speeds up the video but not the audio. " - This is not a problem for Vegas. This envelope works only ON the video element. It is a Velocity Envelope for the Video section of the Event - period. But you do raise a very interesting point, and that is this: How would Vegas know exactly and by what amount to speed up the Audio as a result of an increase in Video velocity? And subsequently, as the VeloEnvel is infinitely variable - speed ups and slow downs and SLIDES in that variation too! - how would Vegas cope with the same variations WITH the audio?
"
Then the subject seems to be talking with no voice." - Correct. You have obviously "hit" a section in your project where exactly the result I pointed out above is occurring. The Visuals are out of sync with the Audio. Or am I missing something?
"
Maybe after increasing the velocity I need to shorten the clip, too. That way, any music or audio won't sound sped up." - Unless I have misunderstood your wished outcome - speeding up a speaker visuals and to remain in sync, then no! IMO, this is the way of madness

. Sure, if you DO have an underlying non-synch dependant music track, fine. But for keeping a speaking in sync with their own audio? The ONLY way I can think of, sensibly, speeding up the visuals AND keeping the audio in sync is to do the Ctrl Squeeze-Event method (
grab the edge of the Event and squash the Event while having the Ctrl key pressed). Now, having done this, you WILL experience a higher register in the voice of the "speaker"

. In which case I :
#1 - Right Click on the Audio and select Properties
#2 - Locate the section "Time Stretch/pitch shift" and disengage the Lock to stretch box.
#3 - From the "Stretch Attributes" drop down box go ahead and pick off the one that is most suitable for your requirements.
This is a fine
FINE function that SONY have created for our purposes. I have used it often when needing to "brighten-up" what was a dreary V/O - when in actual fact WHAT they were saying was being missed - and its importance - by HOW they were saying it! By applying a small judicial amount of speed-up won the day.
Regards - Grazie