A 21: From my "lists of stuff"...
Bard
Virtuoso 1, ecl 8, Complete Adventurer
Fochlucan Lyrist 1, ecl 11, Complete Adventurer
Dragonsong Lyrist 1, ecl 8, Draconomicon, functions like bardic music but different name, does not stack
Warrior Skald 1, ecl 6, Races of Faerun
Spellsinger 1, ecl 6, Races of Faerun
Troubadour of Stars 1, ecl ?, Book of Exalted Deeds
stacks for bardic music uses per day
Seeker of the Song, ecl 11, Complete Arcane
Sublime Chord, ecl 11, Complete Arcane, add only half level
Fochlucan Lyrist , ecl 11, Complete Adventurer
War Chanter, ecl 6 (maybe 5), Complete Warrior
Dawncaller, ecl 7, Races of Stone
Dirgesinger, ecl 6, Libris Mortis
Stormsinger, ecl 6, Frostburn
Lyric Thaumaturge, ecl 7, Complete Mage
special mention
Extra Music, feat, Complete Adventurer, 4 extra uses
Harmonius Knight paladin sub level (Champion of Valor; web enhancement) grants inspire courage, but not as a bardic ability
Initiate of Milil, feat, Champions of Valor, lets you stack bard/cleric/paladin levels to determine what kind of music you have
Initiate of Hlal, feat, Dragon Magic, gain one additional use of bardic music if you already have it
Divine Prankster, ecl 6, Races of Stone, uses bardic music to fuel "Comedic Performance" which has effects similar to bardic music
The Artist, regional feat, Player's Guide to Faerun, 3 extra bardic music uses
Evangelist, ecl 6, Complete Divine, stacks for strength of bard songs, see tex