How can I generate commerce file audio preview automatically?
This sounds like an awesome companion module that could be created and would be popular. From a bit of Googling, you can use SOX or FFMPEG-PHP and the exec() command to actually trim the audio file. From there, it's just a matter of saving this audio file to a new field. I would not use rules unless you need to expose it in the interface for some reason. You could easily achieve the programmatic saving of an audio field (probably a specialized file field) using any of a number of examples out there on the web.

Found an example here: https://stackoverflow.com/questions/7884960/ffmpeg-php-code-example-to-trim-mp3-file

&lt;?php
exec('$ sox input.wav output.wav trim 0 10');
?&gt;