I’ve been hunting for a solution to this for Movie Studio HD 10. I finally started digging in the internal preferences (Shift+Click Preferences on the Options menu and click the Internal tab) and found the answer. This may well apply to Vegas Pro too…
In the Internal Prefs panel, look for eDefEnvAudioASRType (type “env” in the Show Only text box to narrow down the list). The default in VMS-HD 10 is 2, which means a fast log fade… why oh why did they pick this as the default for audio?
Change the value to your preference from the table below. I use smooth, which is the default for video and gives good-sounding audio fades in/out and constant-level crossfades.
2 = fast log
1 = linear
-2 = slow log
4 = smooth cubic
-4 = sharp cubic
eDefEnvVideoASRType is the corresponding setting for video track fades.
Hope this helps! It’s certainly saved me changing lots of fades manually…