Given the other applications you chose to look at, I'm surprised you didn't also look at Audacity. It uses wxWidgets, which is a nice C++ cross-platform GUI framework. It wraps native controls on each platform it supports, and also has a "Generic" variant where it draws its own. It's not very heavy as these things go, and has a nice python binding called wxPython.
I'd check it out before rolling my own :)