I wanted to share a solution that MAGIX Support steered me in the right direction. BTW, they were speedy in replying!
The problem was .mp3 files were being read by the mcplug2.dll and not the mp3plug2.dll, AND I couldn't export to .mp3 as the entire option wasn't available.
I tried the MANY common suggestions of including CTRL+SHIFT Click to reset defaults, deleted the AppData\Local\VEGAS Pro\20.0\ directory, and even reinstalled Vegas. None of this worked!
BACKGROUND
My Laptop installation worked but not my Desktop. I was being nosy with looking at the contents of some files and discovered the Laptop's contents of %AppData\Local\VEGAS Pro\20.0\FFCache_x64_1033.ini file had this:
[mp3plug2]
Path=C:\Program Files\VEGAS\VEGAS Pro 20.0\FileIO Plug-Ins\mp3plug2\mp3plug2.dll
ModTime=32855374
Surrogate=0
{145D078B-8A70-46A8-ACF1-00AA006BA2BA}=MP3 File Format
[mp3plug2.{145D078B-8A70-46A8-ACF1-00AA006BA2BA}]
Access=48
Flags=0
StreamType=1
[{145D078B-8A70-46A8-ACF1-00AA006BA2BA}]
DialogName=MP3 Audio
LangID=1033
ModTime=32855374
Reader={D40B9A2F-F4FD-4D5E-85EE-6D7B8E1D8C2B}
ExtReader=*.mp3
Renderer={ADFA6A4B-A99B-42E3-AE1F-081123ADA04B}
ExtRenderer=*.mp3
The Desktop's version of this file stopped at the Surrogate line:
[mp3plug2]
Path=C:\Program Files\VEGAS\VEGAS Pro 20.0\FileIO Plug-Ins\mp3plug2\mp3plug2.dll
ModTime=32855374
Surrogate=0
I actually copied the Laptop's contents into the Desktop's file after Surrogate=0, but it didn't work...
ALSO, the Desktop's version of file fileio_x64.log (same directory) doesn't have
an entry for .mp3, but the Laptop's does:
INFO 2023-01-22 22:57:52 Loading cached reader|renderer plugin 'MP3 File Format'
from path 'C:\Program Files\VEGAS\VEGAS Pro 20.0\FileIO
Plug-Ins\mp3plug2\mp3plug2.
THE SOLUTION
Support suggested that I enable a Windows Administrator account (from a command prompt opened as administrator, type "net user administrator /active:yes") then switch users and login with this account and run Vegas and see if it worked which it did! I could play .mp3 in the timeline and export to .mp3. He suggested that I "copy the desktop icon for the software. Once you do that, you can then log back into your normal account and paste the desktop icon there to launch it." Copy/paste didn't work, but that didn't make sense to me anyways because the shortcut would still go to the same place which is "C:\Program Files\VEGAS\VEGAS Pro 20.0\vegas200.exe"
THIS GOT ME THINKING THOUGH!!!
I copied the Administrator's %AppData\Local\VEGAS Pro\20.0\FFCache_x64_1033.ini file to my regular account's %AppData\Local\VEGAS Pro\20.0\ directory and IT WORKED!
I disabled this newly enabled account by running "net user administrator /active:no" from a command prompt opened as administrator.
I don't know what caused the problem, but this is a different solution than anything I've seen in trying to fix this issue. So if you have .mp3 issues and none of the other solutions work, give this a try!