Is there a place on the internet where I can enter music notes on a staff and have it played back to me?

I’m not sure whether there are websites that allow you to do this, but there definitely are applications that allow you to do this. A free one that I recall having installed a long time ago was Rosegarden.

Software such as Noteworthy Composer lets you do this. You can also voice the instrumentation. There’s a function to print out your music in standard notation and a MIDI interface so you can actually play music on an electronic keyboard and have it stored digitally to be manipulated by the software. I haven’t tried that feature myself.

I use jamstudio.com. It’s chord based which may be a help or hindrance, depending on your needs.

www.noteflight.com

:D