On Linux this can already be done.
For example on Debian, click on the top left corner of the Mixer window and select "Always on Top".
In the Feature Request page of the wiki there is:
Assuming that this will eventually be possible on Windows, it will then be useful to be able to dock the Transport Toolbar into the Mixer Board window.Mixer Board:
- Needs to stay on top with main window. (13 votes) Although it gets in the way if you have a lot of tracks, you can resize it horizontally. Making it disappear when you use the main window is not the answer. That means shortcuts don't work when it has focus (or developers have to add separate shortcuts for it). Tiling doesn't help if you want to work with maximised windows.
On Linux it is often convenient when working on large projects to move the Mixer Board onto a separate Desktop (multi-Desktop environment), but it is then necessary to switch to the Desktop with the main Audacity window in order to play, pause, stop. It would be a lot more convenient if the Transport Toolbar could be docked in the Mixer Windows AND in the main Audacity Window.
It would also be useful to see the "master" mix level also.