Good question! Though it is quite hard to do in real life, you'll find that in game it is not as difficult as it first seems.
The trick to playing music in the game is knowing three things: Where to find abc files. Where to Put abc files. And what "/play <filename> [index#] [sync]" means.
You can either download our concert collection from the file library (Gallery tab>filelibrary>Music.zip), or search the inter net for more.
There are lots of sites out there that offer abc files, its easy to find some with a simple google search. One of the best, for easy reference is "thefatlute.com". This site in praticular even gives a quick guide on HOW to make abc files. But that is not what this guide is about. On this site you can find songs in the 'Song Book' and even search for song by title or artist. Once you find a song you can get it by either clicking on it and choosing "put all parts into cart" then dowload your cart as a .zip file (quick and free registration required, downloading music and everything else is free on site). Or if you don't want to register, you can copy paste each part from their respective boxes on the song's page and put them into note pad then save the file and change its name in windows explorer from "whatever.txt" to "whatever.abc" it will work like the dowloaded verision.
Although built in with the game, the abc player function, (on some computers) it does not automatically create the folder where you have to put the abc files to work. This is on every computer (no matter where you installed) "C:Documents and Settings*Your Username on Computer*DocumentsThe Lord of the Rings OnlineMusic" As I said, the Music folder is often missing from The Lord of the Rings Online folder upon install so if its so you'll have to creat it yourself (right click in Lord of the Rings folder>new folder>Rename to Music).
Once you have the Music Folder all you have to do is put the abc files in there. (If you dowloaded them as a .zip, first unpack the zip and copy only the .abc files that were in it.)
Now comes the easiest part (unless you mis spell something, that happens, don't get scare if it does, just check your spelling).
Simply equip an instrument (lute, clarinet, etc) and type "/music". At this point your character should have the instrument in their hands and a blue cast bar should be on the screen somewhere with "playing" written on it. If this isn't so, check if you equiped you instrument and type "/music" again. Once you see the blue bar and you have your instrument in hand only one thing remains.
What you need to do depends on if you are playing in a band or alone.
If alone you symply type in "/play file" where "file" is replaced by the name of the .abc you want to use. For example: You have Morrowind.abc in your Music folder, you play it with "/play morrowind" (note that you type "morrowind" and not "morrowind.abc".
If in a group you have to make sure two things: One: You are in a fellowship or raid with the rest of the band. Two: Other than what you do solo, you need to ad two more things to your command line. An index number and the sync command. This means that instead of "/play morrowind" you type "/play morrowind 1 sync". the "1 sync" part never changes so you don't have to worry about that part. Onlything that remains after this is wait for the fellowship/raid leader to type "/playstart" which will make evryone in the fellowship/raid start at same time syncing the song together.
Congratulations! Now you can use .abc files! Go forth and conquer the minds of the people! Long Live the Dread Lord Phinglophin!