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