@arbitrabbit

Try installing shortcutd, it allows you to use the proximity sensor or the camera button as a key and you can assign the task manager to one of them. For example, I have mine set on short press on the camera (don't worry, they won't work while the camera app is open). So I can press the camera button to go to the task manager.

Volume can be changed in settings, it's seperate from the system volume so it can get quite loud if you have them both cranked up..