If you go without having prebooked accommodation then prepare to be disappointed as festivals on this scale usually mean most, if not all, accommodation will be taken.
The only possibility is then to travel some distance to an outlying part of town.
Go to Trip Advisor's Fes travel forums and make enquiries as you will get a far quicker reply which may just save a lot of time and trouble; this also has a hotel's review section which is very useful.
Flights may only be available from London Gatwick with Atlas Blue.

http://www.morocco-fezfestival.com/concert_program5th.html

http://www.tripadvisor.co.uk/Tourism-g293733-Fes-Vacations.html

The only one I know of is Fes Festival of World Sacred Music from 4-12 June 2010.
Going into Moroccan restaurants, cafes or such will get you music any time of the year but ask someone Moroccan in your hotel or riad for the best places to go.