I'm a hobbyist musician; for quite a while, I used a program for Mac's called 'Logic Pro', apparently this is the industry standard application for music creation. (Indeed, at my College if you don't have it at home you can't do any music courses)
So, is there anything like it for Ubuntu?
Please please PLEASE don't say "Audacity"; Logic Pro does so much more then edit audio (for example, I can quite easily create entire songs, minus the lyrics, by selecting instruments and their notes.) and Audacity is...well...laughable in comparison.
Linux MultiMedia Studio
is an easy to use but powerful piece of software for creating music.
It is included in Ubuntu Studio, which I highly recommend you to use as it bundles many other useful software for audio creation and editing. You can install Ubuntu Studio using the ISO image like you used to install ubuntu or you can install it from your existing intallation via the ubuntustudio-desktop package.
Ubuntu Studio comes bundled with lots of other software for media creation (video editing, graphics etc.) and also sets a new default theme. You may not want all of these things if you are primarily interested in audio software. For this reason, Ubuntu Studio is split up into a set of packages. For audio software you can install the ubuntustudio-audio package.
Sounds like you're looking for a synthesiser. Ubuntu Studio (the version of Ubuntu that's for doing audio, video, animation, etc) has a list of synthesisers on https://wiki.ubuntu.com/UbuntuStudio/PackageList So maybe try installing fluidsynth and qsynth (the GUI for fluidsynth) and see how that is?
Here are a few multitrack recording applications:
linux-sound.org has links to various audio production software, and you may also want to look into Ubuntu Studio.
Jokosher is another very popular, user-friendly multi-track sound mixing/recording setup.
MuseScore is what I use for music notation. http://musescore.org/
The features are comparable to that of the notation program Sibelius.