extracting the features of music

Hi every body.
I want to know how can I get the features of a music by audacity? consist of pitch, tempo, amplitude and so on.
I need to change the rhythm or tempo of a music. How can I do it?

Not sure what you mean. The pitch, tempo and amplitude are all there in the music. What do you mean by “extract the features”? What are you trying to do?

The tempo can be changed with the effects:
Change Tempo (fast processing, but sounds echoey when slowing down the tempo)
Change Speed (like slowing down a record/tape, both the tempo and pitch change)
Sliding Time Scale / Pitch Shift (slow processing, but generally much better sound quality than the “Change Tempo” effect when slowing down the tempo)

For more information, see: http://manual.audacityteam.org/man/index_of_effects_generators_and_analyzers.html