I recommend using SoX like this:
sox <input> <output> tempo 0.5
This slows down
<input>'s tempo by a factor of 2 and record the result to
You can add option
--show-progress to display relevant information and progression percentage.
Note that if
<input> is for instance
half-tempo.ogg, SoX will detect the different audio encoding by itself (for more control on that part, read
tempo algorithm should give similar results to VLC's
scaletempo module. However you can try the alternative
sox <input> <output> stretch 2
The result is expected to be more synthetic (again, read
man sox for details) and be aware that the parameter is the inverse of the one given to
2 instead of
0.5 in this example).
SoX offers even more possibilities of time manipulation through
bend options that can be easily explored.
To install SoX using
sudo apt-get install sox
To enable extra codecs (including MP3), add this library:
sudo apt-get install libsox-fmt-all
As a final note, I would come back to VLC since you can play your file slowed down from command line this way:
cvlc --rate 0.5 <input>
So there may be a way to ask VLC to save the result to some file, or to output audio to JACK and then use a JACK compatible recorder.