Hi!
I have found a simple an effective way to change Tempo (and not pitch) of any music recorded in mp3:

Input it in Audacity 1.2.6 (for free in http://audacity.sourceforge.net/), then use the Effect Menu option: "Change tempo". It is not real time and works only on a selected part (or selected all) of the file. Once it is modified, you can play it directly from Audacity at the new Tempo or you can make a new mp3 out of it.

I think this feature may be useful for instruments students as it allows to play along a recording at different speeds. So it is for me and it is not longer necessary to re-write the score in Music Notation SW format to learn/improve on fast pieces.

That is fabulous!
So, you can take a recording and converet it to several different speeds.
File each one separately like
Bach Sonata no1 at 72bpm
Bach Sonata no1 at 86bpm
Bach Sonata no1 at 108bpm
And then play along with which everone as you get better.
This could be a great tool for teachers to use with their students.
Thanks,
Jose.

Audacity allows for changes in percentages, and also in bpm, from x --> y bpm.

The problem with commercial or other peoples' recordings is that you may not know the bpm in advance. You can measure it rather easily with a metronome and a little ear, easier if you have the score in front. Then you can compute the change necessary to the one desired.

I tested the feature it with my own mp3 and simple music I had at hand. I also tested it with the scherzo of the ninth symph Beethoven's. I reduced the tempo to -50% and listened to it. There is some distortion present but still quite usable. And 50% less is a huge change!

A different feature available is the ability to change pitch without changing tempo. It could be very useful to play along ancient instruments like the Traverso, that sounds very low for our modern flutes (about A=420 in a recording I have). For a student to tune so low requires lots of efforts and untunig the flute by using a too long HJ section. I can do this, but my teacher practically has forbidden me to do it because she says it hurts my normal embouchure and scale perception. And sure she knows!
That could be just my own case, but adapting a piece to A=442 with the "change pitch" feature would be easier in any case. The problem here is that the change is given in notes ("from C to C#". for example). With some testing I believe we can easily find the equivalence to A = 442. But haven't tried it so far.


If you make more tests, please post the results here.


<Added>

I forgot to mention that Audacity works with any input accepted, not necessarily mp3 files. You can input almost anything using the "stereo mixer" of the sound card in the PC or even using a microphone. Once it is in Audacity format, all the special effects can be applied (though not in real time). You can then output the modifed file to mp3 or WAV or play it directly from Audacity without needing to export anything.

Jose Luis, thank you for this.

I have music editing software that I bought a couple of years ago, it's called Magix Music Maker. I bought it so that I could record myself and play back duets. It has been pretty complicated, so I haven't used it much.

I did, however, find a way on that program as well to change tempo without altering pitch. This has been amazingly helpful with one of our Symphonic Band pieces this term, which is an absolute BEAR to follow. The recording of the piece (Adam Gorb's Awayday) that I have is by the US Marine Band, and they go fast...way too fast to follow at first.

The slower tempo can sound a bit garbled, but can be tremendously helpful to put other instruments' voices in an ensemble into your own understanding of a part.

Thanks for providing a free software link.