Please let me know how to add mp3 file or player to K2 content. I am using JBG music on my site.

I think the AllVideos plugin from JoomlaWorks will do this for you as well as videos

but mp3 songs don't play. Please let me know how to do it.

Its been a while since ive looked at it but i think that you need to use allvideo and type the plugin code into the module or item body and it will function as a player
check out the joomlaworks forums as i remember seeing it there