Solved Audio ducking plugin?

I make videos of motorcycle life with a live voiceover and would like to know if there's a plugin for what I understand to be audio ducking.
This is for when there are gaps in the voiceover and I've added a music track, so I'd like the music to dynamically rise and fall depending on the volume of my audio track, i.e. when I'm talking to reduce the music volume.
Is anyone aware of a plugin for this method, or is there another way to achieve this?
Cheers.
Edit:
I found SpeakUp by Audified which does exactly this, however it's expensive at $139. The website says it's compatible with Sony Vegas, but I'm not sure if that includes MSP 17.
I'll try the free 30 day trial and see if it works.

VST Audio FX plugins should work in both Vegas Pro and Vegas Movie Studio Platinum.
They would work best in an audio editing program like Sound Forge Audio Studio or Sound Forge Pro.

You can also manually change/vary the volume of Audio Tracks in Vegas over time, by adding an Audio Volume Envelope.
This gives you the ability to raise/lower audio volume continuously.

I have an old tutorial on how to do that here:
www.moviestudiozen.com/free-tutorials/sony-movie-studio-platinum/61-how-to-edit-audio-volume-a-pan-envelopes-using-sony-vegas-movie-studio-hd-platinum-10

That's a great tip and very useful tutorial, cheers.
Btw, MSP didn't pick up the Speakup plugin when I went to preferences/vst.

I'm not an expert when it comes to VST audio plugins.
VST plugins are designed to work in DAWs first - only some will work in Video Editors.
However, some VSTs that don't show in Vegas using default settings, may work by doing this trick.

Go to Options/Preferences/VST Effects
In the alternate VST folder location, add this address:
C:\Program Files\Steinberg\VstPlugins\

Please note that I cannot guarantee this will work, but it is worth trying!

Thanks for the tip, however I don't have the folder \Steinberg.
By the way, I followed your audio envelope video and have now applied the method to a video with voice over and music.
It's very intuitive and does exactly what I needed.
Cheers!