Is the PC connected to the internet? The MP3 license should activate right after Vegas' initial install, but maybe a glitch happened. Otherwise try resetting Vegas.
@john_dennis, thanks for your input, but my audio tracks aren't muted. The project is all audio, no video. I actually wish that was the problem as it would be an easy fix! 😀
Hold down the CTRL + SHIFT keys at the same time and click the executable file <.exe> or shortcut icon to start the program . A pop-up option asks,, "Do you want to reset all Vegas Pro preferences to default values?" and "Delete all Cached Application Data?" This will restore VP to the Factory Default Settings.