I use KDE, so it works a bit differently than what you have, but I sometimes have the same problems. Sometimes a program gets installed that doesn't place its entry in your menu system where you expect it to be (or, for manual installs, sometimes doesn't get added to the menus at all.)
Neither of these packages are installed by default on my system, so you may need to open your package manager and install them if they are not already installed. That should automatically add them to your menus as part of the install process.
The next thing to do is see if your start menu has any kind of search option and use that to find your application. The two apps you mention are established ones and should be in the menu in a category called something like Multimedia.
For your two applications, you shouldn't need to do this, but you can always edit your start menu and add entries for any application you want, even for something as simple as a shell script (which you may learn to write in the future). I do this all the time, but I can't give you the specifics because what I do in KDE won't match your desktop.
Hate to keep bringing up the command system, but regardless of what's going on in your menu system, if you open a terminal and just type
brasero &
OR
sound-juicer &
That's all you need to start it (gui and all) if it's installed correctly (and, if it isn't, you'll get some error messages that are invisible from the gui - which you can use to search the web for a solution or to post to a forum like this to get help for your specific issue.)
The "&" at the end of the command isn't strictly necessary, but it does free up the terminal so you can use it for something else without first closing brasero or whatever other program you are running.
As for music libraries, I would strongly recommend reading up on how the various music players like Amarok, Clementine, Rhythmbox manage your collections. Or, just try them out with a very small collection and make sure everything works as desired before adding a lot more material.
If you figure out a good way to organize all your music folders and all the id3 or other tags in your music files before you start building a large collection, it will save you a ton of time from having to reorganize or re-tag the collection later. I learned this one the hard way.
You should also check this out with regard to any standalone music players like mp3 players you may have. The ones I use primarily organize everything using the id3 tags and when those are messed up, you can end up with a bunch of tracks from multiple albums listed as by Unknown Artist playing in alphabetical order instead of in the original order.
One other note (IMHO): If possible, store your music (and, especially video) files somewhere other than in your root or home partition.
These things use a lot of storage and tend to keep growing as you add more content. If they get too big, they can fill up the partition they're stored on. If it's root, then Linux can run out of room and become non-functional. If it's home (and home isn't on the same partition as root), then your applications and some of the system programs may not have room to work and that's almost as bad.
I make my root and (separate) home partitions relatively small (around 30GB each) and take all the space that's left and make it a partition just for data. If that fills up, it fills up, but it doesn't stop anything else from working.
