I use Adobe Audition to create playbacks, both of artificial sounds and biological ones embedded in silence etc. As I have no experience with Audacity I cannot tell whether the additional cost is justifiable.

I think the search terms you are looking for are "Granular Synthesis". You should find a lot of software to produce sounds for bioacoustics using this technique.

You can also try Praat (praat.org), where you can start from a formula.

If you are familiar with some basic programming, Csound can do about anything. It is not a good program for editing sounds through a time line, but for generating sounds I cannot think of anything Csound cannot do. This includes all kinds of FFT, Ganular Synthesis, waveshaping and additive/subtractive synthesis.

www.csounds.com - free and open source.

Try using chuck (if you're familiar with structured programming) or csound (if you like opcodes and similar ideas). And if you're not familiar with computer programming you may want to try reaktor prism from native instruments.

I vote for (and use) opensource, free, platform-independent Audacity -- http://audacity.sourceforge.net/

I use matlab myself. it helps to have a baseline template of what you are trying to mimic. if you have that, its short order to synthesize and emulate. I do it for fish all the time. Octave is the freeware version, I'm sure theres enough in it to do the same.