I just love the unusual things that people do with Audacity
This one is right up my street, it’s the kind of weird thing I do all the time.
Yes it’s a little time consuming, but here’s how I approach it:
First create a “palette” of sounds - these are the individual “notes” that you will use.
In your case they will in fact be “notes”, but for variety you could add in some noise, or other multi-tone sounds.
For a “conventional tune” you will need to generate each note of the scale - for sine waves, use “Generate → Tone”.
Create about 30 seconds for each pitch that you require (you will need at least 8 for a full octave)
For a conventional scale, you can look up the required frequencies here: http://www.phy.mtu.edu/~suits/notefreqs.html
Now you need to cut some bits off each of these to make your crotchets, quavers, etc.
If you prefer to have long “pad” type sounds, just make a copy that is long enough for the full duration of the longest instance and you will be able to shape the amplitude using the envelope tool.
You may wish to shape the attack and decay of each clip, which you can do with the envelope tool.
When you have completed each distinct sound clip, you need to render them. Try and group them with some sort of system, such as an octave scale of crochets on one track, an octave scale of quavers on a second track, etc.
You can save a bit of time by using the “Change Pitch” or “Change Speed” effects (I’ll not go into that now, but try them out so you are familiar with what they do - you can certainly use these for making more octaves, but may be easier to do that in the next phase.
Right, so you’ve done a lot of work so far, and you don’t want to loose it, so “Save” the project, and “Export” the tracks which have your “notes”.
Now create a new project and “Import” the wav files that you previously exported.
MUTE THESE TRACKS.
Now you can create a melody by copying and pasting your notes into a new track.
It is probably easiest to use a tempo that allows you to use “snap to …” for easy placing of notes - for example, set the “Time format” to 24 frames per second, and use a tempo of 180 bpm, then you have exactly 8 frames per beat. You can always speed it up or slow it down later.
If you need your notes to overlap, use 2 (or more tracks) so that there is no overlap of notes on any single track (as pasting over existing audio will not automatically “mix” the notes).
Now repeat this for any other parts.
Render the tracks for each “instrument” separately (probably best to save a backup of the project before you render).
You can now pan each instrument as required, add effects, or whatever, and mix down your completed masterpiece.
Hope that gives you some ideas (DO have a play with these techniques so that you are familiar with how they work before you get too involved in a big project)
(The original Doctor Who theme was performed on an ARP Odyssey monophonic synthesizer - fantastic instrument )