I just recently upgraded to Vegas Movie Studio HD Platinum 10. I've used version 9 for a long time, and I never had any issues with encoding videos with MP3 audio. I just upgraded to Windows 7, however, and minor compatibility issues on version 9 ultimately pushed me to upgrading to version 10.

I'm using Windows 7 Home Premium. While I did have minor issues with version 9, rendering (even with MP3 audio) was absolutely flawless. In version 10, however, I'm unable to select MP3 audio whenever I'm configuring my rendering options. Here's the process that I'm taking:

1) Choose Project > Render As...

2) In the Render As dialog box, choose "Video for Windows from the "Save as type" drop-down box and click on Custom...

3) In the Custom Settings dialog box, click on the Audio tab.

4) On the Audio tab, choose "MPEG Layer-3" from the "Audio format" drop-down box.

Here's where I run into issues. "No supported formats" is the only option that appears in the "Attributes" drop-down box. Vegas Movie Studio HD Platinum is properly activated and registered, and choosing Help > About Vegas Movie Studio Platinum, then clicking on the Components tab reveals that the MP3 plug-in is properly detected. It displays as follows:

mp3plug2.dll
version: 3.0.0.8680
path: C:\Program Files\Sony\Vegas Movie Studio HD Platinum 10.0\FileIO Plug-Ins\mp3plug2\mp3plug2.dll

What could be preventing me from using MP3 audio in renders?