Hello,
I have few hundreds imported tracks in Audacity. Now, I want to keep 50% length of tracks from the start.
For example: if length of track is from 0 —> 10 seconds. After processing, the length of track is from 0 —>5 seconds. How do do it with multiple tracks. Thank you very much ! 
I presume that the files are different lengths?
What format are the files and what format do the trimmed files need to be?
Audacity has a simple batch processing mechanism called “Chains”. It has some strict requirements / limitations (such as, all the files must be in the same folder). Have a play with Chains - do you think this will work for you if we can adapt the “Trim / Extend” plug-in to work with percentage length?
See these links:
http://wiki.audacityteam.org/wiki/Nyquist_Effect_Plug-ins#Trim_.2F_Extend
http://manual.audacityteam.org/man/chains_for_batch_processing_and_effects_automation.html
Hello,
Yes, the files are different lengths. The format of files is mp3 and mp3 is format of trimmed files. Thank you for giving above links
Are you aware that the length of MP3s is imprecise?
One of the limitations of the MP3 format is that it does not define the overall encoder/decoder delay. A consequence of that is that there is always a few milliseconds of padding at the start of an MP3 file. Will that matter to you?
Another issue is that MP3 is a “lossy” compressed format. The encoding method discards some of the information in order to reduce the file size, which results in some loss of quality, This quality loss is cumulative - every time the audio is encoded, a bit more sound quality is lost, and it is impossible to recover the lost sound quality. Audacity needs to decode MP3s before it can work on them - the files are decoded on import, which means that when exported as MP3 they must be re-encoded, which reduces the sound quality. Will that matter to you?