Does it have to be a library?
Or can you control NE555, NE556 chips that are setup as astable oscillators?
http://www.mouser.com/ds/2/389/CD00000479-103226.pdf
http://www.mouser.com/ds/2/389/CD00000480-103598.pdf
See figure 16 as one way to set it up - trigger a tone for a length of time.
Or replace the output of the first with the arduino output.
Set up as many oscillators as you want, use shift register outputs to control them.
Take all the tone outputs, run into a op-amp summing amplifier.
See NE555 Figure 15, use either waveform for different sounds.