The thing is, I read in the web, that if I decode mp3 I have to purchase a licence.
A license is needed for commercial (i.e., revenue-generating) use of mp3/mp3PRO in broadcast systems, streaming applications, other content distribution systems or for use of mp3/mp3PRO on physical media. However, no license is needed for private, non-commercial activities, not generating revenue or other consideration of any kind or for entities with associated annual gross revenue less than US$ 100 000.00. (link
So in your case, you won't need a license