Hello.
I don't know that it API you use to produce your audio but the first simple methods to set up are to play with the volume and pitch of the sound (with a multiplier in range of 0.8 to 1.1 for example). To go further, if your API offers more options, you can try adding different effects like spatialization or reverberation. If you have enough knowledge in physics you can also go further and simulate Doppler effects! But in general just playing with the volume and the pitch is enough to break the monotony :3
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I used Bfxr, and just smashed the Randomize button a few times.
I have no idea about spatialization, reverberation, or Doppler effects, but they sound awesome and I really want to learn how to use them! But most likely, I'll just have a friend of mine test that for me. I'm extremely bad when it comes to sounds...