Okay, to get this working nicely in 15.04, I recommend doing the following:
Add two extra lines to the "limits.conf" file
1) paste the following into a terminal:
sudo pluma /etc/security/limits.conf
2) navigate to the bottom of the file and paste the following two lines into it:
@audio - rtprio 90
@audio - memlock unlimited
3) save and close the file.
Make a bash script to load Rosegarden in the correct way
1) Firstly, you need to knock up a little bash scrip to start rosegarden in a particular way.
a) open a blank pluma file
b) paste the following text into it:
c) save the file as rosegarden.sh
2) use caja to navigate to your newly created file. it should be in your home folder.
3) right click the file and choose "properties"
Make sure you select the "allow executing the file as program" check-box, in the "permissions" tab
Then close the properties dialogue box.
4) You can, from now on, if you wish, simply start Rosegarden by double clicking this file. However, if you like things all nice and neat, like me, then follow the instructions, below, to make it so you can run this bash script from your Rosegarden entry in your main menu.
a) go to your main menu and right click and choose "edit menus"
b) navigate in, the menu editor, to where your Rosegarden launcher is. Then, double click it.
c) press the "browse" button and navigate to where your rosegarden.sh bash script is located and then select it. The path to the script will now appear in the command field. You can now close the launcher and close the menu editor.
From now on, whenever you select Rosegarden form the menu, it will actually be running this script to load it, in which case it will work properly
Edit to add:
Of course, the Rosegarden instructions only apply if you actually choose to install Rosegarden!
However, that said, and irrespective of whether you install Rosegarden, I would still strongly recommend you implement the instructions on adding the extra lines to your "limits.conf" file.