I've been tossing around an idea of an adventure short that would require all bard PCs, but I don't think we have enough bards of equal level to pull it off. The adventure would require a party of adventurers to act as foreign liaisons to a city-state where the population communicates through song accompanied through instrumental play akin to Jack Vance's Moon Moth short story. The city population in that story were the equivalent of evil bards, where status was given by daring, musical talent with extremely difficult instruments, and skill with the sword when all else failed.