I wrote it in Python, because it would take me the least time to write it. My challenge was to write this without referencing any other code, like Audacity. This made it more fun.
The GUI isn't very good, specifically because there are no other dialogs, but I have most of the important features covered: Add Silence Before, Add Silence After, Chop, Append File, Mix File, Modulate, Make Louder, Make Softer. And, it loads and saves standard wav files (16 bit or 8bit), so it is actually practical for editing things you have recorded.
Then, I added a lot of audio effects. I would read the gist of an effect on Wikipedia and try to code it. This was even more fun, because if I got something wrong, it sometimes played very strange sounds. (I don't have any analog-style effects, like filters yet, but this is something I would really like to learn). In the GUI, you can use the following effects: Reverse, Octave up/down, Change pitch (the most challenging to think of how to write), Vibrato, Flanger, Echo, Reverb, Chorus, and Alien Voice. The alien voice modulates the samples with a sine wave - I came up with it while experimenting and don't know why it sounds strange.
Listen to these (the wavs were generated by my program):
Next, I added code to synthesize sounds. Sine, triangle, square, and sawtooth waves aren't that great by themselves, so I tried combinations of them to get the most interesting voices. The secret to making a sine wave sound better is to quietly mix in frequencies at that are slightly different. Surprisingly, the subtle beat frequencies make it sound expressive and musical. I worked a long time to create an electric organ-like instrument out of sine waves, and finally came up with something.
Here are the results:
Finally, I put all of the pieces together. I had an audio editor that can modify and arbitrarily change the pitches of sounds. So, I made a sampler! I wrote a command-line interface for a program where you can type in notes and play them. It parses commands like a4 into the note A440, or 4.f#5 into "play a dotted quarter note at F# 5". As an instrument, you can use anything - samples of your voice, a sample of a real instrument, your computer's MIDI, or even the same synthesized instruments like Electric Organ or Smooth.
The program works well (the effects even work in both 16bit and 8bit sounds).
Lest you think this program is polished, I must admit that it is not. This is not a joke, it is actually in the source:
manualsynth(audiodata, nsamples, _extend([[freq, .5*0.9**(8-1)]], [[freq*(i+1),.5*0.1*0.9**((8-1)-i)] for i in range(1,8)],[[freq*1.00606, .5*0.9**(8-1)]], [[freq*1.00606*(i+1),.5*0.1*0.9**((8-1)-i)] for i in range(1,8)]))
I should get around to fixing that sometime.