Im addition to my long career in museums as a conservator I have also produced live musuc events and was the executive director if one of the largest international non-profit membership organizarions for songwriters and composers for several years. I also created and teach an online course on, Copyright and Intellectual Property 101 for Museums".

First, there are three performing rights organizations (PRO's) that license music in the United States. So getting a blanket or live performamce license from ASCAP would only legally cover you for songs that are represented from ASCAP, and not BMI or SEASAC, the two other songwriting PRO's. So to be in compliance with the US copyright law you would have to get licenses from all three. Second, it does not matter if the performers are beginners, part-time, full time, or on tour. What you are licensing in the copyright of the creator(s) of the song, the songwriter. Current copyright law duration is for the life of the creator(s) plus 70 years. After that the song goes into public domain where anyone can use it freely. There are some exceptions to this, especially for works created before 1979. So, any live performances of music created before 1923 is generally in the public domain. So traditional and a lot of classical music would fall under that. What you most have to be concerned about are when performers cover songs under active copyright. The PRO's do not take the performers to court, they go after the venues and those who put on the event. Even if you required performers to only perform they created, you are still liable - all of the PRO's have an online self-reporting feature for their members to "get paid" for gigs. That is how they catch small venues all the time. The live performance licenses are based on the size of your theater, auditorium, or your revenue. The larger the space the higher the cost. If you only do a few events a year you might try calling them to negotiate the cost. Usually they won't but it's worth a try. There is always a lot of myth about non-profits and fair use. The copyright law applies to both profit and non-profit organizations. Fair use can br complex because it is more a set of guidelines where the specifics are defined by decisions in court. Generally a judge will look at whether the use of a copyrighted work is commercial. If you charge for your music events, if you charge admission when there is live music, if you sell food in a cafe with live music, that is commercial use. Another test is how much of the work is used. For instance, was only a small section of a song used or was it the entire song. Fair use expressly exempts the use of copyrighted works for criticism, commentary, satire, parody, personal (not public) use, and some limited educational uses such as in classrooms or libraries. Some activities of museums fall under Fair Use, but live music is not one of them. 

There is a lot more involving copyright and the use of music in museums and it gets more compkex with the other copyrights involved (mechanical rights, sound recordings, etc) so I've only just replied about live music here.

Cheers!

Dave