1) A running copy of Nexus Media Importer (NMI) is required to access files
on any microUSB connected storage but does not allow other apps to access those files.
2) NMI only allows reading of certain (media) file types on external storage and will "pipe" or direct the file stream to any app you select. For example, you click a .MP3 file and tell NMI to use WinAmp to play the data streamed.
3) unless you can get NMI to send a playlist to a compatible player, only one 'file' can be played at a time. This is not an issue for a movie file since it can be piped to MXplayer, say, and will play for over an hour but it is a pain to play music 3 minutes a pop.
4) BUT you can use a program like http://www.shchuka.com/software/mergemp3/
to combine as many mp3 files as you like into one
in a few seconds and then play that one file.
5) you can use NMI to transfer files to your local storage and then use them however you like but that storage is READ ONLY so you can not move file TO the external drive from the N7.
If you root your N7 and employ stickmount, you may play media files (and all other files) however you like and move them back and forth as you like.