To regain a mastery you must repeat the quest to unlock it. Mastering a second ‘song’ negates the first mastery. Caution, only one ‘mastery’ can be held at any time. They are found in the Conservatory of Music in Britain (both facets).
The ability to cast ‘spellsongs’, bard mastery, is achieved by performing one of three quests given by Sir Berran the Song Wielder, Sir Hareus the Battle Rouser or Sir Falean the Spirit Soother.