Secret Rabbit Code (aka libsamplerate) is a Sample Rate Converter for audio.
One example of where such a thing would be useful is converting audio from
the CD sample rate of 44.1kHz to the 48kHz sample rate used by DAT players.

SRC is capable of arbitrary and time varying conversions; from downsampling
by a factor of 12 to upsampling by the same factor. Arbitrary in this case
means that the ratio of input and output sample rates can be an irrational
number. The conversion ratio can also vary with time for speeding up and
slowing down effects.

SRC provides a small set of converters to allow quality to be traded off
against computation cost. The current best converter provides a
signal-to-noise ratio of 97dB with -3dB passband extending from DC to 96%
of the theoretical best bandwidth for a given pair of input and output
sample rates.

Homepage:
http://www.mega-nerd.com/SRC/
