I usually receive the emails from jammr site with the links for recordings download and I can get two files. One is a m4a file with the mixed jam and the other is a zip file with individual tracks. The first shows only the date and hour or the jam in the title and the second contains even the members of the band. But if I want to share the only m4a file I cannot find the info with band members for credits. I can edit by myself using an editor but how can I recover the name or the jammers if I have only the m4a file at the moment? (I tested with vlc media player using tools/media info menu).

There is no metadata in the file so you'd only have that information from the recorded jam email notification.