Why would you hear "startup music"? Do you mean the boot chime (that you would hear when you start or restart your Mac?)
That's NOT the same as standby, or sleep. When you wake up a Mac that is sleeping, you shouldn't normally hear anything at all.
What you describe sounds like your MBPro is actually shutting off when you close the lid, when it should be just sleeping.
When you have that problem, try closing the lid again. With it closed, wait for 15 seconds or so. Then, reopen.
Does your screen light up then?
If you get the same problem, and can only "fix" it by restarting, then that's just not working properly. It could possibly be a hardware fault, so your local Apple store can fix for you.
Or, you can also revert back to Yosemite, as the problem may simply be a bug in your El Capitan beta. You can report that to Apple. Revert back to Yosemite first, just to make sure that the problem is only in the ElCap beta.