Am I A Part Owner If I Fix a Defective Audio File?

Dear Rich: Some friends made an audio recording of a discussion. They could not get the audio file to work so they they asked me to convert it to an MP3 file so it would actually be playable. So I helped them convert this to a tangible product and they asked me to keep a copy of it for them. They then copyrighted it without my knowledge and sued me for infringement. My question is since I helped make this a tangible working file for them to copyright am I not considered a joint owner especially if I have the written consent between us to convert it for them?
No, converting an audio file from one format to another or fixing defects that make a file unplayable do not amount to copyrightable contributions and you wouldn't acquire any ownership rights to the resulting work. As a result of a 1991 Supreme Court case, copyright law no longer rewards labor (or "sweat of the brow" efforts), only original contributions to works of authorship.
Sued for infringement? We're missing something, here. Why were you sued for infringement? If you only possessed the converted copy and were not distributing it, there are no damages and no unauthorized copying. So there's no basis for someone to sue.