Here's a link to the Book of Phyz resources for sound and waves.
I also showed Pasco's WavePort at the PTSOS2 workshop. It generates sound waves that you (or students) can manipulate. A waveform appears on the screen and a corresponding sound comes out of the computer's speaker. Onscreen handles allow for the manipulation of the wave's amplitude and wavelength.
You can also use WavePort to generate multiple sound waves at once (think interference and beats).
WavePort also has an "oscilloscope" function for sampling sounds from the computer's microphone.
WavePort is Mac + PC. You can download a free demo of the software at Pasco's site
The physics simulators (also Mac + PC) from The University of Colorado's PhET group are free. They have nice simulators in the areas of sound and waves, as well as mechanics, heat, electromagnetism, light, and even chemistry!