Secret Rabbit Code (aka libsamplerate) is a Sample Rate Converter for
audio. One example of where such a thing would be useful is in
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. The
conversion ratio can also vary with time for speeding up and slowing
down effects.