A while back when I purchased an earlier version of Sony Vegas Movie Studio, Acid Express was part of the "bonus" software. The most recent version of SVMS didn't offer it, but it is still there for free and is up to version 7.
"ACID Xpress is a free, 10-track version of ACID software for music recording, editing and mixing. It's easy to use, even if you've never tried loop-based music creation before. Just pick some loops, paint them into a track and play them back. Any loops you choose work together because ACID Xpress matches the tempo and pitch automatically. When you're ready, publish your song to ACIDplanet.com." -- http://www.acidplanet.com/downloads/xpress/
This version 7 has inline midi editing which allows you to use the built-in midi keyboard to create music and sound effects. You don't even need to open a file.
1). Create a midi track: CTRL-ALT-Q
2). Enable inline midi editing: Q
3). Maximize track height by clicking the maximize button in the upper left corner of the track. That will reveal a portion of the midi-keyboard. You can also drag the bottom border of the track to heighten it. There is another midi keyboard down in the lower right of the screen, but you won't be using that for this.
4). Just below the red record button and mute button and solo button trio near the top in the track header is a button that says Piano 1. If you click on that, you will see a list with Sound Effects near the bottom of the list. Click on that item and you will see another list with various sound effects. You can use the draw tool and paint tool to draw/paint the notes in the grid.
5.) After you have created your masterpiece, you can render an MP3 or WMV or WAV or PCA: File > Render As. In the free version 7, there is a bitrate limit of 128 mbs for MP3.
That's all I've figured out so far.