do you know a free software that convert M4A to MP3?
i found some but they didn't really worked coz they were trial versions.

schabernack
1
lan
"Free MP3 WMA converter" might be useful - I wouldn't install the rest of the stuff it's bundled with, if it asks, just say no.
Haven't tried it, but it's freeware, and the few reviews I've read seem to suggest it works.
2
zook
This one is free and should do the job.
It converts pretty much anything into anything and has been very dependable for me.
Pro tip