As the DM, I would handle the singing thing like this:
Bard: I sing a song to calm the crowd down.
DM: Yeah? Whatcha singing about?
Bard: Um, I sing a song about dopplegangers infiltrating the church and murdering all the priests.
DM: Hm. That doesn't exactly impress the crowd: someone in it yells out how YOU'RE infiltrating their peaceful city and murdering them. That's gonna be a hefty penalty on your diplomacy check.
Bard: Wait! Wait! I sing a song about a famous war started over a simple misunderstand! The song emphasizes the needless bloodshed that resulted from people acting before they knew what happened, and the many grieving families that resulted! It's a terribly sad, quiet song!
DM: Hmm. Okay, I'm feeling nice: forget the doppleganger song. Yeah, that song's enough to get you a bonus on your diplomacy check.
One of my favorite bardic moments involved a situation where, given some downtime, I spent the afternoon at a schmancy cafe singing a song that metaphorically represented the troubles in the city from a viewpoint very favorable to us PCs. When, that night, a wizard used illusions and enchantments to try to persuade a crowd of nobility to kill us, I was able to start singing the song I'd spread earlier in the day, gaining a very hefty bonus on my checks due to the crowd's familiarity with its themes (I'd rolled a critical success on my earlier perform check, IIRC).
That's how bards should work: they should know the best tools to influence people, and they should look for as much synergy as possible between these different tools.
Daniel