I don't think it is possible, as you are auditioning mixed wma file. That "mixer", in my view is a redundant feature from days when Library was a bunch of non-modal windows.
The way I would do what I think you are trying to do is:
1) "Plus" sign by Demo button in Library, check Double click plays chords, not pre-made demo
2) Make mixer "float" resize to your needs. It will stay on "top" of library.

[Linked Image from i.postimg.cc]


Does this help?